The party itself is about an hour, ending with the fireworks and buffet. The family pool closes on the Magic and the Wonder (not sure about the Dream but if it's not closed, the area will be packed!). Since they also set up an evening buffet, the decks are crowded.
There will be programming in the kid's clubs, movies in the movie theater, and some other events in a location or two - but since most of the cruise staff is involved in the party, there's not a lot else going on. My recommendation would be to find some recent navigators online for the same cruise itinerary that you are planning to sail, and see what is listed at the same time as the party.