I think of stuff like this.
Student or Learner
If you just hear "field day" what do you think of?
Is it a field trip or sports day? Or something else?
It reminds me of nothing except the phrase/idiom "He's having an absolute field day with ...". I readily admit that, until today, I have never bothered to wonder what a field day is!
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
Pope of the Dictionary.com Forum
The original meaning was a sports day at a public (BrE) or private (AmE) school. I imagine it is still used that way occasionally, but I agree with emsr2d2 that the metaphorical meaning she gave is the most common usage tofay.