Last year we got there at 11 the night before and had a cozy little camp fire. All the slobs didn't show up until a half hour before shooting time. A group of three snuck into the bushes 75 yards away and proceded to shoot at the first flock of the day working the spread. They shot at anything within 100 yards, including birds right over top of us that we didn't even shoot at because they were too high!
This year I think we are going to go the evening before just to get everything set up and to gather some wood while it's still light. Do we need to go that early, no but it's kinda a fun tradition we started. First day you obviously want to get their earlier than you normally would. I would think 3am would be good for the first day if your worried about your spot, but I'm sure some guys might get there the night before. It really depends on where your at, how public it is, and the number of "waterfowlers" or skybusters are in your area.