1) Of the days, I think Friday is the best option. Teams have Thursday evening to shake down any robot issues, and my experience is that many teams do not stay over the night after the competition ends.

2) The attraction of a social for me was as a pace-changer, so I'm all in favor of holding a social off-site, be it a team-run social in a rented out hotel's hospitality room, or in a local Science Museum.

3) My opinion is that 10:30-11:00 is a good time to close up the social.

4) I'd expect a team-run social to have access to food (Burger King, Krispy Kreme, etc... down the street), but for a FIRST organized social, I think a free buffet is in order.

5) Entertainment-wise, I think the main purpose of the social is to socialize; the secondary purpose is to eat food, and the tertiary purpose is to enjoy the activities. Entertainment is nice, but I'd rather the money be put toward the buffet.
A LAN-party has been mentioned quite a bit in this thread (7 times) though, and while I'd prefer a DJ, I can see how a massive Halo 2 tournament could compete with that. Why not ask your regional event supervisors if they could set a room aside for a LAN party, and then have the party be BYOC (Bring your own computer)? If you had enough people interested, I don't see why it would be an issue (except maybe for power requirements depending on the size of the party).

So what does it come down to? Quoting Greg Marra here, "fun is funner than no fun." So no matter what, try to have fun.