A dead dog party is the traditional winding-down party after a major fandom-oriented event, most notably after a science fiction convention.
The dead dog party is generally very low-key, as, after a several-day long event, few of the attendees are likely to have huge amounts of energy - hence their name. The party is an attempt to ease people back into the real world outside the world of the convention, particularly for the organising committee, and can be an effective method of warding off the depression which is often associated with the end of a major event.
Analogies can be drawn to the decompression parties following Burning Man events and - by extrapolation - to the athlete parties following the closing ceremonies of Olympic Games.