My understanding is that the number of tickets/visitors is set and basically the same for the entire run, so the actual OBB party crowds should pretty similar no matter when you go. Crowds in the parks around the dates you choose might be another matter though. We went last year to OBB on Tuesday 10/25 and had a blast. Easily got through all the treat trails and Villains Grove and did a TON of rides with barely any lines. I'd go again for sure. The crowds in the parks before and after that date were insane though. Weather is tough to call but more likely to be hotter in September.