We've been a few times in July, August and September. I don't really see any difference in the weather. It's very hot and humid but if you go prepared it really is not that bad. We never spend the entire day in the parks. We go early for rope drop (or try to anyway) and then we stay until we are ready to have lunch. We will either eat in the park and then go back to the resort or go back to the resort and have lunch there. Then we relax either in the room or go to the pool for a few hours before heading back to the parks around dinner time. We will usually stay until it closes or until we poop out....which ever come first. The perk of summer visits is that the parks are open very late. Very rarely will we go to the parks 2 days in a row. Usually it's one day on, one day off. Also, we like to spend a few days at the waterparks. They are a lot of fun and it never feels hot there.
It is super crowded around the 4th of July....like shoulder to shoulder crowded. The crowds are lowest in the last week of August through September.