A day for manoeuvres and tactical exercises in "the field".
A school day for athletic events; a sports day.
A day of class taken away from school for a field trip.
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.
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.
(specifically US Navy, US Coast Guard and US Marine Corps) A day on which there is top-to-bottom all-hands cleaning.