I know we use to go the third week in August..it was hot. It's likely the same in September. However, the parks are open late..and also it was less crowded end of August because the Florida schools had already started. We don't start until after Labor Day in our district, so taking the kids out wasn't an issue. If it weren't for the free dining that's been offered the last few years, I would say that Sept. used to be less crowded, but that hasn't been my understanding...although again, I've not gone during that time period. In fact we would not choose to go during that time period with the "free dining" promotion that runs for that time period.
We did have a reservation during the last week of "free dining"(end of Sept..into first week of October) and actually cancelled..because I was unable to secure a reservation in in any of the restaurants that we wanted to dine in That was at three months out. Disney did call and tell us they were keeping the restaurants open later, and there were openings at 10:00 at night. While we don't have little ones(our youngeset is 16) and we don't mind eating late..eating dinner at 10:00 just wasn't something we wanted to do. If you are planning a trip during the free dining promo, I would recommend that you make your reservations at the six month out point, as we waited until three months out( and I thought that was early enough),our dining options were limited. Between that and not wanting to deal with the "free dining" promotional crowds..we cancelled, and went the following May. We were staying in a Deluxe resort the "free dining" isn't as much of a value, as you are paying full rack rate. It's a much better value, and perhaps worth dealing with some of the inconvenience if you are staying in a moderate, and an even better value if you are staying in a value resort. That not being so in our situation, We opted to wait. I received a package pin code mailer, and saved 30% the following May, and had no problems at all making ADR's. It was our first time using a package and dining plan and we enjoyed it.