May 21 (Eastern Orthodox liturgics)

• Holy Equals-to-the-Apostles Emperor Constantine (337), and Empress Helena, his mother (327)[1][2][3][note 2]
• Martyrs Polyeuctus, Victorinus, and Donatus, at Caesarea in Cappadocia.[5][6]
• Hieromartyr Secundus and those with him, in Alexandria (356)[2][3][5]
• Saint Adelphios, Bishop of Onouphis, in Byzantine Egypt (c. 362, or c. 431)[7][note 3]
• Saint Boros, monk.[8][note 4]
• Saint Christopher I, Patriarch of Antioch (967)[9]

• Hieromartyrs Timothy, Polius and Eutychius, deacons, in Mauretania Caesariensis (Morocco).[5][10][11]
• Hieromartyr Valens and Companions, Bishop who was martyred in Auxerre in France along with three children.[5][11]
• Martyr Secundinus, in Cordoba in Spain under Diocletian (c. 306)[5][11]
• Saint Hospitius the Hermit of Nice, Wonderworker (581)[2][3][5][11][12]
• Saint Barrfoin (Bairrfhionn, Barrindus) of Killbarron (6th century)[11][note 5]
• Saint Gollen (Collen, Colan) of Denbighshire, a saint who has given his name to Llangollen in Wales (7th century)[11]
• Saint Isberga (Itisberga) of Aire, reputed sister of Charlemagne, nun at Aire (Aria) in the Artois, of which she is the patroness (c. 800)[11]
• Saint Ageranus (Ayran, Ayrman) of Beze, a monk at Bèze in France, martyred by the Vikings (888)[11]
• Saint Theobald (Thibaud), Archbishop of Vienne (970-1001)[11]

• Blessed Constantine (Yaroslav), prince, and his children Michael and Theodore, Wonderworkers of Murom (1129)[2][3][13]
• Saint Cyril II (Kirill), Bishop of Rostov (1262)[2][14][15]
• Saint Basil of Ryazan, Bishop (1295)[2][3][16][note 6]
• Saint Helen of Decani, Serbia, Princess (c. 1350)[2][3][17]
• Venerable Cassian the Greek, monk of Uglich, Wonderworker (1504)[2][3][18][19]
• Saint Agapitus of Markushev (Vologda) the Hieromartyr, Abbot (1584)[2][3][20]
• Saint Constantine the Fool-for-Christ, of Novotorzanin, Russia (c. 16th century)[21][note 7]
• New Martyrs King Constantine Brancoveanu of Wallachia and his four sons Constantine, Stephen, Radu, and Matthew, and his counsellor Ianache (1714)[22][note 8]
• Martyr Pachomios of Patmos (1730)[3][23]

• Uncovering of the relics (1998) of St. Andrew Ogorodnikov, fool-for-Christ of Simbirsk (1841)[2]
• Repose of Elena Pavlova of Chepel, Kharkiv (1885)[2]
• Repose of Elder Isaac of Dionysiou Monastery of Mount Athos (1932)[2]
• The Anastenaria fire-walking ritual is performed from May 21–23 in some villages in Northern Greece and Southern Bulgaria, coinciding with the feast of Constantine and Helen.[25][note 9]
• Synaxis of all saints of Karelia,[26]

• 
  Sts Constantine and Helena, Holy Equals-to-the-Apostles.
• 
  Sts Constantine and Helena, Holy Equals-to-the-Apostles (Mosaic in Saint Isaac's Cathedral, St. Petersburg, Russia).
• 
  Stained glass window ( 1986 ) showing St Collen (Llangollen, Wales - St.Collen parish church)
• 
  Icon of the Saints of Murom.
• 
  King Constantin Brâncoveanu of Wallachia, and his sons (Church mural in Hurezi Monastery).
• 
  “Vladimir” Icon of the Most Holy Theotokos.
• 
  Mosaic icon of the Virgin of Tenderness, with the eponym "Episkepisis"("The Visitation"). From Triglia in Asia Minor, Church of Saint Basil (late 13th century)

• May 21/June 3. Orthodox Calendar (ORTHOCHRISTIAN.COM).
• June 3 / May 21. HOLY TRINITY RUSSIAN ORTHODOX CHURCH (A parish of the Patriarchate of Moscow).
• Dr. Alexander Roman. May. Calendar of Ukrainian Orthodox Saints (Ukrainian Orthodoxy - Українське Православ'я).
• May 21. Latin Saints of the Orthodox Patriarchate of Rome.
• May 21. The Roman Martyrology.

Greek Sources

• Great Synaxaristes: (in Greek) 21 ΜΑΪΟΥ. ΜΕΓΑΣ ΣΥΝΑΞΑΡΙΣΤΗΣ.
• (in Greek) Συναξαριστής. 21 Μαΐου. ECCLESIA.GR. (H ΕΚΚΛΗΣΙΑ ΤΗΣ ΕΛΛΑΔΟΣ).

Russian Sources

• (in Russian) 3 июня (21 мая). Православная Энциклопедия под редакцией Патриарха Московского и всея Руси Кирилла (электронная версия). (Orthodox Encyclopedia - Pravenc.ru).
• (in Russian) 21 мая (ст.ст.) 3 июня 2013 (нов. ст.). Русская Православная Церковь Отдел внешних церковных связей. (DECR).