Julian calendar

The ordinary year in the previous Roman calendar consisted of 12 months, for a total of 355 days. In addition, a 27- or 28-day intercalary month, the Mensis Intercalaris, was sometimes inserted between February and March. This intercalary month was formed by inserting 22 or 23 days after the first 23 days of February; the last five days of February, which counted down toward the start of March, became the last five days of Intercalaris. The net effect was to add 22 or 23 days to the year, forming an intercalary year of 377 or 378 days.[5] Some say the mensis intercalaris always had 27 days and began on either the first or the second day after the Terminalia (23 February).[6]

According to the later writers Censorinus and Macrobius, the ideal intercalary cycle consisted of ordinary years of 355 days alternating with intercalary years, alternately 377 and 378 days long. In this system, the average Roman year would have had 366+1⁄4 days over four years, giving it an average drift of one day per year relative to any solstice or equinox. Macrobius describes a further refinement whereby, in one eight-year period within a 24-year cycle, there were only three intercalary years, each of 377 days (thus 11 intercalary years out of 24). This refinement averages the length of the year to 365.25 days over 24 years.

In practice, intercalations did not occur systematically according to any of these ideal systems, but were determined by the pontifices. So far as can be determined from the historical evidence, they were much less regular than these ideal schemes suggest. They usually occurred every second or third year, but were sometimes omitted for much longer, and occasionally occurred in two consecutive years.

If managed correctly this system could have allowed the Roman year to stay roughly aligned to a tropical year. However, since the pontifices were often politicians, and because a Roman magistrate's term of office corresponded with a calendar year, this power was prone to abuse: a pontifex could lengthen a year in which he or one of his political allies was in office, or refuse to lengthen one in which his opponents were in power.[7]

If too many intercalations were omitted, as happened after the Second Punic War and during the Civil Wars, the calendar would drift out of alignment with the tropical year. Moreover, because intercalations were often determined quite late, the average Roman citizen often did not know the date, particularly if he were some distance from the city. For these reasons, the last years of the pre-Julian calendar were later known as "years of confusion". The problems became particularly acute during the years of Julius Caesar's pontificate before the reform, 63–46 BC, when there were only five intercalary months (instead of eight), none of which were during the five Roman years before 46 BC.

Caesar's reform was intended to solve this problem permanently, by creating a calendar that remained aligned to the sun without any human intervention. This proved useful very soon after the new calendar came into effect. Varro used it in 37 BC to fix calendar dates for the start of the four seasons, which would have been impossible only 8 years earlier.[8] A century later, when Pliny dated the winter solstice to 25 December because the sun entered the 8th degree of Capricorn on that date,[9] this stability had become an ordinary fact of life.

Although the approximation of 365+1⁄4 days for the tropical year had been known for a long time,[10] ancient solar calendars had used less precise periods, resulting in gradual misalignment of the calendar with the seasons.

The octaeteris, a cycle of eight lunar years popularised by Cleostratus (and also commonly attributed to Eudoxus) which was used in some early Greek calendars, notably in Athens, is 1.53 days longer than eight mean Julian years. The length of nineteen years in the cycle of Meton was 6,940 days, six hours longer than the mean Julian year. The mean Julian year was the basis of the 76-year cycle devised by Callippus (a student under Eudoxus) to improve the Metonic cycle.

In Persia (Iran) after the reform in the Persian calendar by introduction of the Persian Zoroastrian (i. e. Young Avestan) calendar in 503 BC and afterwards, the first day of the year (1 Farvardin=Nowruz) slipped against the vernal equinox at the rate of approximately one day every four years.[11][12]

Likewise in the Egyptian calendar, a fixed year of 365 days was in use, drifting by one day against the sun in four years. An unsuccessful attempt to add an extra day every fourth year was made in 238 BC (Decree of Canopus). Caesar probably experienced this "wandering" or "vague" calendar in that country. He landed in the Nile delta in October 48 BC and soon became embroiled in the Ptolemaic dynastic war, especially after Cleopatra managed to be "introduced" to him in Alexandria.

Caesar imposed a peace, and a banquet was held to celebrate the event.[13] Lucan depicted Caesar talking to a wise man called Acoreus during the feast, stating his intention to create a calendar more perfect than that of Eudoxus[13] (Eudoxus was popularly credited with having determined the length of the year to be 365+1⁄4 days).[14] But the war soon resumed and Caesar was attacked by the Egyptian army for several months until he achieved victory. He then enjoyed a long cruise on the Nile with Cleopatra before leaving the country in June 47 BC.[15]

Caesar returned to Rome in 46 BC and, according to Plutarch, called in the best philosophers and mathematicians of his time to solve the problem of the calendar.[16] Pliny says that Caesar was aided in his reform by the astronomer Sosigenes of Alexandria[17] who is generally considered the principal designer of the reform. Sosigenes may also have been the author of the astronomical almanac published by Caesar to facilitate the reform.[18] Eventually, it was decided to establish a calendar that would be a combination between the old Roman months, the fixed length of the Egyptian calendar, and the 365+1⁄4 days of Greek astronomy. According to Macrobius, Caesar was assisted in this by a certain Marcus Flavius.[19]

Caesar's reform only applied to the Roman calendar. However, in the following decades many of the local civic and provincial calendars of the empire and neighbouring client kingdoms were aligned to the Julian calendar by transforming them into calendars with years of 365 days with an extra day intercalated every four years.[20][21] The reformed calendars typically retained many features of the unreformed calendars. In many cases, the New Year was not on 1 January, the leap day was not on the traditional bissextile day, the old month names were retained, the lengths of the reformed months did not match the lengths of Julian months, and, even if they did, their first days did not match the first day of the corresponding Julian month. Nevertheless, since the reformed calendars had fixed relationships to each other and to the Julian calendar, the process of converting dates between them became quite straightforward, through the use of conversion tables known as "hemerologia".[22] Several of the reformed calendars are only known through surviving hemerologia.

The three most important of these calendars are the Alexandrian calendar and the Ancient Macedonian calendar─which had two forms: the Syro-Macedonian and the 'Asian' calendars. Other reformed calendars are known from Cappadocia, Cyprus and the cities of (Roman) Syria and Palestine. Most reformed calendars were adopted under Augustus, though the calendar of Nabatea was reformed after the kingdom became the Roman province of Arabia in AD 106. There is no evidence that local calendars were aligned to the Julian calendar in the western empire. Unreformed calendars continued to be used in Gaul (the Coligny calendar), Greece, Macedon, the Balkans and parts of Palestine, most notably in Judea.

The Alexandrian calendar adapted the Egyptian calendar by adding a 6th epagomenal day as the last day of the year in every fourth year, falling on 29 August preceding a Julian bissextile day. It was otherwise identical to the Egyptian calendar. The first leap day was in 22 BC, and they occurred every four years from the beginning, even though Roman leap days occurred every three years at this time (see Leap year error). This calendar influenced the structure of several other reformed calendars, such as those of the cities of Gaza and Ascalon in Palestine, Salamis in Cyprus, and the province of Arabia. It was adopted by the Coptic Orthodox Church and remains in use both as the liturgical calendar of the Coptic church and as the civil calendar of Ethiopia.

The Asian calendar was an adaptation of the Ancient Macedonian calendar used in the Roman province of Asia and, with minor variations, in nearby cities and provinces. It is known in detail through the survival of decrees promulgating it issued in 8 BC by the proconsul Paullus Fabius Maximus. It renamed the first month Dios as Kaisar, and arranged the months such that each month started on the ninth day before the kalends of the corresponding Roman month; thus the year began on 23 September, Augustus's birthday. Since Greek months typically had 29 or 30 days, the extra day of 31-day months was named Sebaste—the emperor's day—and was the first day of these months. The leap day was a second Sebaste day in the month of Xandikos, i.e., 24 February. This calendar remained in use at least until the middle of the fifth century AD.

The Syro-Macedonian calendar was an adaptation of the Macedonian calendar used in Antioch and other parts of Roman Syria. The months were exactly aligned to the Julian calendar, but they retained their Macedonian names and the year began in Dios (November) until the fifth century, when the start of the year was moved to Gorpiaios (September).

These reformed calendars generally remained in use until the fifth or sixth century. Around that time most of them were replaced as civil calendars by the Julian calendar, but with a year starting in September to reflect the year of the indiction cycle.

The Julian calendar spread beyond the borders of the Roman Empire through its use as the Christian liturgical calendar. When a people or a country converted to Christianity, they generally also adopted the Christian calendar of the church responsible for conversion. Thus, Christian Nubia and Ethiopia adopted the Alexandrian calendar, while Christian Europe adopted the Julian calendar, in either the Catholic or Orthodox variant. Starting in the 16th century, European settlements in the Americas and elsewhere likewise inherited the Julian calendar of the mother country, until they adopted the Gregorian reform. The last country to adopt the Julian calendar was the Ottoman Empire, which used it for financial purposes for some time under the name Rumi calendar and dropped the "escape years" which tied it to Muslim chronology in 1840.

The Tusculum portrait of Julius Caesar

The first step of the reform was to realign the start of the calendar year (1 January) to the tropical year by making AUC 708 (46 BC) 445 days long, compensating for the intercalations which had been missed during Caesar's pontificate. This year had already been extended from 355 to 378 days by the insertion of a regular intercalary month in February. When Caesar decreed the reform, probably shortly after his return from the African campaign in late Quintilis (July), he added 67 more days by inserting two extraordinary intercalary months between November and December.[note 1]

These months are called Intercalaris Prior and Intercalaris Posterior in letters of Cicero written at the time; there is no basis for the statement sometimes seen that they were called "Undecimber" and "Duodecimber", terms that arose in the 18th century over a millennium after the Roman Empire's collapse.[note 2] Their individual lengths are unknown, as is the position of the Nones and Ides within them.[23]

Because 46 BC was the last of a series of irregular years, this extra-long year was, and is, referred to as the "last year of confusion". The new calendar began operation after the realignment had been completed, in 45 BC.[24]

The Julian months were formed by adding ten days to a regular pre-Julian Roman year of 355 days, creating a regular Julian year of 365 days. Two extra days were added to January, Sextilis (August) and December, and one extra day was added to April, June, September, and November. February was not changed in ordinary years, and so continued to be the traditional 28 days. Thus, the ordinary (i.e., non-leap year) lengths of all of the months were set by the Julian calendar to the same values they still hold today. (See Sacrobosco's incorrect theory on month lengths (below) for stories purporting otherwise.)

The Julian reform did not change the method used to account days of the month in the pre-Julian calendar, based on the Kalends, Nones and Ides, nor did it change the positions of these three dates within the months. Macrobius states that the extra days were added immediately before the last day of each month to avoid disturbing the position of the established religious ceremonies relative to the Nones and Ides of the month.[25] However, since Roman dates after the Ides of the month counted down toward the start of the next month, the extra days had the effect of raising the initial value of the count of the day following the Ides in the lengthened months. Thus, in January, Sextilis and December the 14th day of the month became a.d. XIX Kal. instead of a.d. XVII Kal., while in April, June, September and November it became a.d. XVIII Kal.

Romans of the time born after the Ides of a month responded differently to the effect of this change on their birthdays. Mark Antony kept his birthday on 14 January, which changed its date from a.d. XVII Kal. Feb to a.d. XIX Kal. Feb, a date that had previously not existed. Livia kept the date of her birthday unchanged at a.d. III Kal. Feb., which moved it from 28 to 30 January, a day that had previously not existed. Augustus kept his on 23 September, but both the old date (a.d. VIII Kal. Oct.) and the new (a.d. IX Kal. Oct.) were celebrated in some places.

The inserted days were all initially characterised as dies fasti (F – see Roman calendar).[26] The character of a few festival days was changed. In the early Julio-Claudian period a large number of festivals were decreed to celebrate events of dynastic importance, which caused the character of the associated dates to be changed to NP. However, this practice was discontinued around the reign of Claudius, and the practice of characterising days fell into disuse around the end of the first century AD: the Antonine jurist Gaius speaks of dies nefasti as a thing of the past.[27]

The old intercalary month was abolished. The new leap day was dated as ante diem bis sextum Kalendas Martias ('the sixth doubled day before the Kalends of March'), usually abbreviated as a.d. bis VI Kal. Mart.; hence it is called in English the bissextile day. The year in which it occurred was termed annus bissextus, in English the bissextile year.

There is debate about the exact position of the bissextile day in the early Julian calendar. The earliest direct evidence is a statement of the 2nd century jurist Celsus, who states that there were two-halves of a 48-hour day, and that the intercalated day was the "posterior" half. An inscription from AD 168 states that a.d. V Kal. Mart. was the day after the bissextile day. The 19th century chronologist Ideler argued that Celsus used the term "posterior" in a technical fashion to refer to the earlier of the two days, which requires the inscription to refer to the whole 48-hour day as the bissextile. Some later historians share this view. Others, following Mommsen, take the view that Celsus was using the ordinary Latin (and English) meaning of "posterior". A third view is that neither half of the 48-hour "bis sextum" was originally formally designated as intercalated, but that the need to do so arose as the concept of a 48-hour day became obsolete.[28]

There is no doubt that the bissextile day eventually became the earlier of the two days for most purposes. In 238 Censorinus stated that it was inserted after the Terminalia (23 February) and was followed by the last five days of February, i.e., a.d. VI, V, IV, III and prid. Kal. Mart. (which would be 24 to 28 February in a common year and the 25th to 29th in a leap year). Hence he regarded the bissextum as the first half of the doubled day. All later writers, including Macrobius about 430, Bede in 725, and other medieval computists (calculators of Easter) followed this rule, as does the liturgical calendar of the Roman Catholic Church. However, Celsus' definition continued to be used for legal purposes. It was incorporated into Justinian's Digest,[29] and in the English statute De anno et die bissextili of 1236,[30] which was not formally repealed until 1879.

The effect of the bissextile day on the nundinal cycle is not discussed in the sources. According to Dio Cassius, a leap day was inserted in 41 BC to ensure that the first market day of 40 BC did not fall on 1 January, which implies that the old 8-day cycle was not immediately affected by the Julian reform. However, he also reports that in AD 44, and on some previous occasions, the market day was changed to avoid a conflict with a religious festival. This may indicate that a single nundinal letter was assigned to both halves of the 48-hour bissextile day by this time, so that the Regifugium and the market day might fall on the same date but on different days. In any case, the 8-day nundinal cycle began to be displaced by the 7-day week in the first century AD, and dominical letters began to appear alongside nundinal letters in the fasti.[31]

During the late Middle Ages days in the month came to be numbered in consecutive day order. Consequently, the leap day was considered to be the last day in February in leap years, i.e., 29 February, which is its current position.

The Julian reform set the lengths of the months to their modern values. However, a different explanation for the lengths of Julian months, usually alleged to the 13th century scholar Sacrobosco,[32] but also attested in 12th century works,[33] is still widely repeated, but is certainly wrong.

Allegedly according to Sacrobosco, the month lengths for ordinary years in the Roman Republican calendar were a standard lunar calendar, similar the Greek city calendars. From Ianuarius to December, the month lengths were:

Sacrobosco then thought that Julius Caesar added one day to every month except Februarius, a total of 11 more days to regular months, giving the ordinary Julian year of 365 days. A single leap day could now be added to this extra-short Februarius:

He then said Augustus changed this, by taking one day from Februarius to add it to Sextilis, and then modifying the alternation of the following months, to:

so that the length of Augustus (August) would not be shorter than (and therefore inferior to) the length of Iulius (July), giving us the irregular month lengths which are still in use.

Although plausible and filled with ingenious arithmetical organization, there is abundant evidence disproving this theory.

First, the Fasti Antiates Maiores, a wall painting of a pre-Julian Roman calendar has survived.[34][35] That pre-Julian calendar confirms the literary accounts that the months were already irregular before Julius Caesar reformed them, with an ordinary year of 355 days (not evenly divisible into Roman weeks), not 354, with month lengths arranged as:

Also, the Julian reform did not change the dates of the Nones and Ides. In particular, the Ides were late (on the 15th rather than 13th) in March, May, July, and October, showing that these months always had 31 days in the Roman calendar,[36][37] whereas Sacrobosco's theory requires that March, May, and July were originally 30 days long and that the length of October was changed from 29 to 30 days by Caesar and to 31 days by Augustus.

Further, Sacrobosco's theory is explicitly contradicted by the 3rd and 5th century authors Censorinus[38] and Macrobius,[39] and it is inconsistent with seasonal lengths given by Varro, writing in 37 BCE,[8] before Sextilis was renamed for Augustus in 8 BCE, with the 31 day Sextilis given by an Egyptian papyrus from 24 BCE,[40] and with the 28 day Februarius shown in the Fasti Caeretani, which is dated before 12 BCE.[41]

The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days. There is a simple cycle of three "normal" years followed by a leap year and this pattern repeats forever without exception. The Julian year is, therefore, on average 365.25 days long. Consequently, the Julian year drifts over time with respect to the tropical (solar) year (365.24217 days).[42]

Although Greek astronomers had known, at least since Hipparchus,[43] a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed equinox times and the seasons. This discrepancy was largely corrected by the Gregorian reform of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years.[44] (Even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years.)[42]

The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%, making the Julian 10.8 minutes longer. The accumulated effect of this difference over some 1600 years since the basis for calculation of the date of Easter was determined at the First Council of Nicea means for example that, from 29 February Julian (13 March Gregorian) 1900 and until 28 February Julian (13 March Gregorian) 2100, the Julian calendar is 13 days behind the Gregorian calendar; one day after (i.e. on 29 February Julian or 14 March Gregorian), the difference will be 14 days.

The Romans later renamed months after Julius Caesar and Augustus, renaming Quintilis as "Iulius" (July)[3] in 44 BC and Sextilis as "Augustus" (August) in 8 BC. Quintilis was renamed to honour Caesar because it was the month of his birth.[68] According to a senatus consultum quoted by Macrobius, Sextilis was renamed to honour Augustus because several of the most significant events in his rise to power, culminating in the fall of Alexandria, occurred in that month.[69]

Other months were renamed by other emperors, but apparently none of the later changes survived their deaths. In AD 37, Caligula renamed September as "Germanicus" after his father;[70] in AD 65, Nero renamed April as "Neroneus", May as "Claudius" and June as "Germanicus";[71] and in AD 84 Domitian renamed September as "Germanicus" and October as "Domitianus".[72] Commodus was unique in renaming all twelve months after his own adopted names (January to December): "Amazonius", "Invictus", "Felix", "Pius", "Lucius", "Aelius", "Aurelius", "Commodus", "Augustus", "Herculeus", "Romanus", and "Exsuperatorius".[73] The emperor Tacitus is said to have ordered that September, the month of his birth and accession, be renamed after him, but the story is doubtful since he did not become emperor before November 275.[74] Similar honorific month names were implemented in many of the provincial calendars that were aligned to the Julian calendar.[75]

Other name changes were proposed but were never implemented. Tiberius rejected a senatorial proposal to rename September as "Tiberius" and October as "Livius", after his mother Livia.[76] Antoninus Pius rejected a senatorial decree renaming September as "Antoninus" and November as "Faustina", after his empress.[77]

Much more lasting than the ephemeral month names of the post-Augustan Roman emperors were the Old High German names introduced by Charlemagne. According to his biographer, Charlemagne renamed all of the months agriculturally into German.[78] These names were used until the 15th century, over 700 years after his rule, and continued, with some modifications, to see some use as "traditional" month names until the late 18th century. The names (January to December) were: Wintarmanoth ("winter month"), Hornung,[note 3] Lentzinmanoth ("spring month", "Lent month"), Ostarmanoth ("Easter month"), Wonnemanoth ("joy-month", a corruption of Winnimanoth "pasture-month"), Brachmanoth ("fallow-month"), Heuuimanoth ("hay month"), Aranmanoth ("reaping month"), Witumanoth ("wood month"), Windumemanoth ("vintage month"), Herbistmanoth ("harvest month"), and Heilagmanoth ("holy month").

The calendar month names used in western and northern Europe, in Byzantium, and by the Berbers, were derived from the Latin names. However, in eastern Europe older seasonal month names continued to be used into the 19th century, and in some cases are still in use, in many languages, including: Belarusian, Bulgarian, Croatian, Czech, Finnish,[79] Georgian, Lithuanian, Macedonian, Polish, Romanian, Slovene, Ukrainian. When the Ottoman Empire adopted the Julian calendar, in the form of the Rumi calendar, the month names reflected Ottoman tradition.

The Julian calendar was in general use in Europe and northern Africa until 1582, when Pope Gregory XIII promulgated the Gregorian calendar. Reform was required because too many leap days were added with respect to the astronomical seasons under the Julian scheme. On average, the astronomical solstices and the equinoxes advance by 10.8 minutes per year against the Julian year. As a result, 21 March (which is the base date for the calculating the date of Easter) gradually moved out of alignment with the March equinox.

This is a visual example of the official date change from the Julian calendar to the Gregorian.

While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value,[98] it was evidently felt to be of little importance at the time of the Julian reform (46 BC). However, it accumulated significantly over time: the Julian calendar gained a day every 128 years. By 1582, 21 March was ten days out of alignment with the March equinox, the date where it supposedly had been in 325, the year of the Council of Nicaea.

The Gregorian calendar was soon adopted by most Catholic countries (e.g., Spain, Portugal, Poland, most of Italy). Protestant countries followed later, and some countries of eastern Europe even later. In the British Empire (including the American colonies), Wednesday 2 September 1752 was followed by Thursday 14 September 1752. For 12 years from 1700 Sweden used a modified Julian calendar, and adopted the Gregorian calendar in 1753.

Since the Julian and Gregorian calendars were long used simultaneously, although in different places, calendar dates in the transition period are often ambiguous, unless it is specified which calendar was being used. In some circumstances, double dates might be used, one in each calendar. The notation "Old Style" (O.S.) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (N.S.), which either represents the Julian date with the start of the year as 1 January or a full mapping onto the Gregorian calendar. This notation is used to clarify dates from countries that continued to use the Julian calendar after the Gregorian reform, such as Great Britain, which did not switch to the reformed calendar until 1752, or Russia, which did not switch until 1918 (see Soviet calendar). This is why the Russian Revolution of 7 November 1917 N.S. is known as the October Revolution, because it began on 25 October O.S.

Throughout the long transition period, the Julian calendar has continued to diverge from the Gregorian. This has happened in whole-day steps, as leap days that were dropped in certain centennial years in the Gregorian calendar continued to be present in the Julian calendar. Thus, in the year 1700 the difference increased to 11 days; in 1800, 12; and in 1900, 13. Since 2000 was a leap year according to both the Julian and Gregorian calendars, the difference of 13 days did not change in that year: 29 February 2000 (Gregorian) fell on 16 February 2000 (Julian). This difference of 13 days will persist until Saturday 28 February 2100 (Julian), i.e. 13 March 2100 (Gregorian), since 2100 is not a Gregorian leap year, but is a Julian leap year; the next day the difference will be of 14 days: Sunday 29 February (Julian) will be Sunday 14 March (Gregorian); the next day Monday 1 March 2100 (Julian) falls on Monday 15 March 2100 (Gregorian).[99]

Russian icon of the Theophany (the baptism of Jesus by John the Baptist) (6 January), the highest-ranked feast which occurs on the fixed cycle of the Eastern Orthodox liturgical calendar.

Although most Eastern Orthodox countries (most of them in eastern or southeastern Europe) had adopted the Gregorian calendar by 1924, their national churches had not. The "Revised Julian calendar" was endorsed by a synod in Constantinople in May 1923, consisting of a solar part which was and will be identical to the Gregorian calendar until the year 2800, and a lunar part which calculated Easter astronomically at Jerusalem. All Orthodox churches refused to accept the lunar part, so all Orthodox churches continue to celebrate Easter according to the Julian calendar, with the exception of the Finnish Orthodox Church.[100] (The Estonian Orthodox Church was also an exception from 1923 to 1945.[101])

The solar part of the Revised Julian calendar was accepted by only some Orthodox churches. Those that did accept it, with hope for improved dialogue and negotiations with the western denominations, were the Ecumenical Patriarchate of Constantinople, the Patriarchates of Alexandria, Antioch, the Orthodox Churches of Greece, Cyprus, Romania, Poland (from 1924 to 2014; it is still permitted to use the Revised Julian calendar in parishes that want it), Bulgaria (in 1963), and the Orthodox Church in America (although some OCA parishes are permitted to use the Julian calendar). Thus these churches celebrate the Nativity on the same day that western Christians do, 25 December Gregorian until 2799.

The Orthodox Churches of Jerusalem, Russia, Serbia, Montenegro, Poland (from 15 June 2014), North Macedonia, Georgia, Ukraine, and the Greek Old Calendarists and other groups continue to use the Julian calendar, thus they celebrate the Nativity on 25 December Julian (which is 7 January Gregorian until 2100). The Russian Orthodox Church has some parishes in the West that celebrate the Nativity on 25 December Gregorian until 2799.

Parishes of the Orthodox Church in America Bulgarian Diocese, both before and after the 1976 transfer of that diocese from the Russian Orthodox Church Outside Russia to the Orthodox Church in America, were permitted to use this date. Some Old Calendarist groups which stand in opposition to the state churches of their homelands will use the Great Feast of the Theophany (6 January Julian/19 January Gregorian) as a day for religious processions and the Great Blessing of Waters, to publicise their cause.

Most branches of the Eastern Orthodox Church use the Julian calendar for calculating the date of Easter, upon which the timing of all the other moveable feasts depends. Some such churches have adopted the Revised Julian calendar for the observance of fixed feasts, while such Orthodox churches retain the Julian calendar for all purposes.[102]

The Ancient Assyrian Church of the East, an East Syriac rite that is commonly miscategorised under "eastern Orthodox", uses the Julian calendar, where its participants celebrate Christmas on 7 January Gregorian (which is 25 December Julian). The Assyrian Church of the East, the church it split from in 1968 (the replacement of traditional Julian calendar with Gregorian calendar being among the reasons), uses the Gregorian calendar ever since the year of the schism.[103] The Syriac Orthodox Church uses both Julian calendar and Gregorian calendar based on their regions and traditions they adapted.

The Oriental Orthodox Churches generally use the local calendar of their homelands. However, when calculating the Nativity Feast, most observe the Julian calendar. This was traditionally for the sake of unity throughout Christendom. In the west, some Oriental Orthodox Churches either use the Gregorian calendar or are permitted to observe the Nativity according to it.

The Armenian Patriarchate of Jerusalem of Armenian Apostolic Orthodox Church uses Julian calendar, while the rest of Armenian Church uses Gregorian calendar. Both celebrate the Nativity as part of the Feast of Theophany according to their respective calendar.[104]

The Julian calendar is still used by the Berbers of the Maghreb in the form of the Berber calendar.[105]