Field Day Meaning

(US military, specifically US Navy, US Coast Guard and US Marine Corps) A day on which there is top-to-bottom all-hands cleaning.

Example: A school day for athletic events; a sports day.
A day of class taken away from school for a field trip.
  They went to the park and had a field day playing on the swings.
  The reporters will have a field day with a comment like that.
  The scandal was a field day for the press.