Definition from Wiktionary, the free dictionary
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.
- (military) A parade day.
- 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.
- 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 and US Marine Corps) A day on which there is top-to-bottom all-hands cleaning.
- have a field day (idiomatic)