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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- (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 ….
- A school day for athletic events; a sports day.
- A day of class taken away from school for a field trip.
- (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.
- (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.
- (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)