Last year we went the last week of August & left the Saturday of Labor Day weekend. We were dreading the hot weather, but they had a cool front come through. The first part of the week it was only in the mid 80's and it got up to 90 the day before we left, so you never know. It was way warmer at home that week.
The other thing that was weird was that there was hardly anyone at the campround. We were in the 900 loop on that trip (wanted to try the Premium sites out), and we were one of three people on the loop until later that week. There was no one in the 1000 loop at first, no one in the 200 loop, and only 4 in the 100 loop. 600 seemed to be pretty full, but I did not like the new expansion of it where there were no trees.