Wordorigins.org

How did field day come to mean a time of great success and opportunity? Field would seem to be an odd choice at first blush, but as with many such terms, an examination of its semantic development makes all clear.

Field day originally referred to a day of military exercises. From A Scheme for Equipping and Maintaining Sixteen Men of War, from 1747:

These periodical Intervals of eating and drinking...are to the Citizens as it were Field Days, for improving...their Valour.

By the early 19th century, the term had generalized to mean a day of big events. From an 1827 letter by Thomas Creevey:

Saturday was a considerable field day in Arlington Street,...and a very merry jolly dinner and evening we had.

Finally, by the early 20th century the big events had become great successes. From E.F. Norton’s 1925 Fight for Everest:

The two experts, who had for days been working every afternoon, and often late into the night, put in a regular field-day.

(Source: Oxford English Dictionary, 2nd Edition)