field day

Apparently the idiomatic usage is derived from the "parade day" military use. A parade is much easier than the soldiers’ usual drilling and forced exercise.

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field day (plural field days)

1. (military) A day for manoeuvres and tactical exercises in "the field".
   • 1937, Siegfried Sassoon, Sherston's Progress, London: Faber, page 621 (in The Complete Memoirs of George Sherston ):

     This morning I got up, with great difficulty, at 6.30, and at 7.45 we started out for a Brigade Field Day. Did an attack from 10.30 to 2.30, but it wasn't a strenuous one for me as I was told to "become a casualty" soon after the 3000 yard assault began ….

2. A school day for athletic events; a sports day.
3. A day of class taken away from school for a field trip.
4. (idiomatic) A great time or a great deal to do; a period of bustling activity.

   They went to the park and had a field day playing on the swings.

5. (idiomatic) A great time or a great deal to do, at somebody else's expense.

   The reporters will have a field day with a comment like that.

   The scandal was a field day for the press.

6. (US military, specifically US Navy, US Coast Guard and US Marine Corps) A day on which there is top-to-bottom all-hands cleaning.

• have a field day (idiomatic)

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