field day

Meanings

1. A day for maneuvers and tactical exercises in the field (across the landscape).
2. A school day for athletic events.
3. A day of class taken away from school for a field trip.
4. A great time or a great deal to do; a period of bustling activity.
5. A great time or a great deal to do, at somebody else's expense.
6. A day on which there is top-to-bottom all-hands cleaning.

Etymology

Some postulate the idiomatic usage is derived from the "parade day" military use. A parade is much easier than the soldiers’ usual drilling and intense exercise.