One day out of the week (generaly Thursday) when you are forced to clean everything in your room or squad bay like it was fresh out of the department store. Your work is then inspected by someone with higher rank (who's house has probably never been "field day'd") and it is this persons responsibility to find any spec of dust, wrinkled sheets, and/or pubic hair in and around the most ridiculous places you could imagine.
JJ: We have field day tonight.
Sean: What? Tell everyone I died man, I'm dipping out!