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