欧博What are the origins of the phrase "field d
According to phrases.org.uk, field day has a meaning of
A day of excitement or a circumstance of opportunity
with the first reference being in 1747, in A Scheme For Equipping And Maintaining Sixteen Men Of War: Of Twenty Guns Each
...as it were Field Days, for improving and approving their Valour and Prowels...
As you mentioned, this is often used with the idiom "Have a field day". Two things stand out to me here:
1) A field day, in a militaristic sense, refers to the opportunity to both prove and improve ones ability to accomplish required tasks. This could refer to battlefield maneuvers, but it could also refer to more simplistic tasks (such as cleaning the barracks).
2) As a field day originally referred to a chance at practicing, the difficulty involved should be minimal. This would draw a clear connection with, for example, reporters being handed a story that is easy to write about. In my opinion, it is considered a field day because the task at hand is uncomplicated and straightforward, requiring a minimal amount of skill in order to succeed.
In combining those two points, field day as it refers to the cleaning of the barracks, is a chance for a unit to show that it is able to accomplish simple tasks efficiently.