We went last Oct on a Sunday and I agree with the school-night theory. (We're doing the first Sun party this year as well.) We went to MK on Sat (afternoon & evening) and again on Sunday before the party, and the party crowd was nothing compared to those. How many tix are sold for this...5,000? I can't remember.
The crowds were very managable, IMHO. And the later you stay, the better it gets! By around 10pm last year we could walk on just about anything.