Mouse party

A celebration by subordinates on the occasion of their supervisor's absence. Usually applied to office workers, but relevant to any mischievous celebration orchestrated by peers answerable to a common individual.

Borrowing from the phrase 'when the cat's away, the mice will play', a mouse party might be celebrated by a team of employees taking a two-and-a-half-hour lunch together, or sneaking out of the office an hour early to drink at a bar.