A “field day” is a day of excitement or the opportunity to do a lot of something one wants to do rather than the usual routine stuff.
Although the phrase makes me think of kids released from their school desks to participate in sports and athletic contests, it’s mostly used today by news media criticizing someone. “The press is going to have a field day if they get wind of this story.” It is also used to describe the experience of being released from one’s usual work schedule, or, I suppose, from any sort of routine.
The term “field day” was first used by the military to mean, literally, a day spent in field maneuvers. The first reference of this type appears to be from the London newspaper The Daily Journal, September 1723. A specific military reference comes a little later, in The Edinburgh Advertiser, May 1776: “The officers, on a general field day, instead of commanding, are obliged to coax them [the soldiery] to go through their different maneuvers.”
During the 1800s, the phrase began to be used for any event that might happen in a field; for example:
— Hunting: “Sometimes a dance (though rarely on field days, for then the gentlemen were rather tired).” This was found in Lord Byron’s Don Juan, 1823.
— Scientific expeditions: “We had a delightful field-day in the abbey.” This quote comes from Sir George Gilbert Scott Recollections, 1878.
Also during the 1800s, the term also began to be used for any exciting or welcome event, as in Thomas Creevey’s Letters, 1827: “Saturday was a considerable field day in Arlington Street … and a very merry jolly dinner and evening we had.”
In modern days, we don’t spend much time in fields and, during the 1900s, the term was further extended to refer to what the tabloids might do with an item of news.
There is, according to Wikipedia, even a Canadian pop-punk band from Calgary, Alberta, called Field Day. But I’d be willing to bet they don’t play the trumpets, pipes, and drums common to a military band.
To each his own! (I’ll take the military pipes and drums, thank you.)