A "party" held on the last day of the convention after the convention has officially ended. There is rarely any programming or supplies from the convention provided for this "party", people are expected to bring along whatever left-over food/drink they've still got that they are not taking home, and otherwise to buy drink from the hotel bar and just unwind and relax.
There are various opinions on the reason for the name, but I'm sure you can use Google if you're interesed :-)
After the convention is "officially" over (the closing ceremony has happened, the metaphorical baton passed on to the next year's convention etc.) a number of fans are still around the convention hotel/site for the rest of the evening. It is traditional for the organising committee of that year's Eastercon to suggest a place for the remaining fans to meet up and socialise. Usually a function room or one of the hotel bars.
A lot of fans go home on the Monday afternoon/evening, but as many as 30% of the membership may stay the extra night (or at least later into the evening on the Monday). Some committees have provided snacks or drinks (usually if they are left over from the convention, though occasionally money will be spent on providing additional refreshments if the convention has cash left), others have provided some sort of video presentation (the masquerade, the play etc.) for those staff and committee that were too busy during the con to be able to attend those events.
The usual pattern for things is that the convention programme ends by around 5pm or so, then there is a quick clear up ("tear down") of all the tech equipment, posters, the various convention offices etc. and then many people go out to eat (including, usually, the committee for whom it will be the first, last and only time to sit down with the (remaining) guests). It is common to appoint a "Duty Committee Member" from the ranks of other convention committees or operations staff to handle the tear down and any crises that may occur while the committee is unwinding at the end of the convention they have worked so hard to provide. People drift back in from their meals at various times in the evening and meet up in the "Dead Dog Party" and sit and chat and relax. There is usually a bar nearby which, if possible, continues to sell drinks at the convention prices rather than the (usually higher) hotel prices.
A lot of equipment and supplies are needed to run a convention. Some are hired in and returned, some are borrowed and returned. But some equipment and supplies are bought to use at the con and sometimes the leftovers are auctioned off at the end of the convention during the Dead Dog Party. This may include unused paper for the newsletter, coffee/tea supplies from Green Room, stationery from Ops, props for programme items etc.