- a day devoted to military exercises and display
- a day of athletic events and contests
- ☆ a day spent in outdoor scientific study
- an occasion of enjoyably exciting events, extraordinary opportunity, or highly successful activity: the press had a field day with the senator's confession
- a. A day set aside for sports or athletic competition, as at a school.b. An outdoor meeting, social event, or festivity.c. A day for military exercises, maneuvers, or display.
- A time of great pleasure, activity, or opportunity: The media had a field day with the sensational story.
(plural field days)
- (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, speficially US Navy and US Marine Corps) A day on which there is top-to-bottom all-hands cleaning.
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.