I've been in May, July, August, October, November, and December. May was nice crowd-wise during the week, busy on the weekend and was quite humid. July was horrendous. I swear the heat index was 115 the day we were at Epcot. If you weren't wet due to sweating, you were getting drenched in the rain storms. Parks were busy the whole time. August was not as bad as July, with only one horrible rainstorm, and it was not AS hot (only in the high 90s). Parks were a little less crowded, but still busy. October we went Columbus Day weekend so it was busy for that, but the weather was nice and comfortable. High 80s. Went this year the day after Thanksgiving and I loved it! Sure the parks were busy on the weekend, but they were still managable. We wore shorts/t-shirts during the day and carried sweatshirts. Personally, I found the weather to be wonderful, but I am a cold weather girl. Nice to walk around and not worry about heat exhaustion!
I would reccommend the week after Thanksgiving to anyone who wants to see the parks for the holidays but not the holiday crowds.
I try my hardest never to go in July again. Heat and tour groups do not mix well!