Having served in the United States Marine Corps, I have often wondered about the origins of the word "field day," but I am not referring to its meaning as
1.a. a day for military exercises or maneuvers
1.b. an outdoor meeting or social gathering
1.c. a day of sports and athletic competition
a time of extraordinary pleasure or opportunity
(as in the newspaper had a field day with the scandal)
(Source: Merriam Webster.)
In the military, or at least in the United States Marine Corps that I once knew, "field day" also refers to (and much more frequently) that one day of the week when Marines clean the barracks. The day used to be Thursday, but in recent times has moved to Monday by some commands.
I actually submitted this question to NPR's "A Way With Words" about a decade ago, but never received a reply back or heard the question answered on the radio. I would love to know when it first began being used as a phrase to mean cleaning the barracks.