|This article is about an older version of DF.|
If you have a Meeting hall designated from a table (dining room), a cage ("zoo"), a well, or a statue (Sculpture garden), dwarves will often throw parties there and many of your precious productive workers will now display the status Attending Party. This has benefits, in that dwarves will get happy thoughts from admiring fine or finely placed statues, cages and the animals in the cages, as well as from making friends and talking to them. They will also slowly develop social skills. However, parties may slow down your fortress's work as your dwarves idly hang-out, sometimes for more than a season.
Oddly, dwarves usually cancel party attendance to get a drink or some food. Despite their numerous desperate attempts they obviously know next to nothing about how to roll in style. Dwarves will also go On Break from a party, even if that means hanging out in the same meeting hall, talking to the same people. Partying is serious work for dwarves.
Parties can be canceled by "free"ing the room where they take place via. Even if you immediately recreate the room the party goers will disperse. Canceling the meeting hall status is not enough.
Each eligible room can have its own party going. With multiple overlapping rooms(like several tables in a dining room set to the same area), each can have its own party. If you free the room a particular party started in, the dwarves attending that party will stop partying, but those partying from other rooms (even in the same space) will keep going. If you're going to overlap rooms that can have parties, it may be best to make each room-designated construction from a different material, so you can free the correct party to get an important dwarf to go back to work.
 Party Reduction & Prevention
Less-social dwarves seem to be less likely to throw parties, and less likely to make friends to invite them. Less-happy dwarves also seem to be less likely to throw parties. Thus antisocial and dour dwarves can be assigned important labors without as much risk of stopping work to party.
Note that dwarves don't throw parties at meeting halls designated from zones. If all your meeting halls are of this type, your dwarves won't party.