We went this year from 9/14 - 9/22. Now, don't get me wrong, we did have a great time. Afterall, it is Disney World and we are Disney fanatics, but it was very crowded and very hot. I am a faithful fan of the UG and if it weren't for knowing about their touring plans and knowing how to utilize the Fastpasss system, the lines would not have been manageable. There was the occasion that you walked up to something in the late afternoon and there was only a 10 or 15 minute wait, but most of the headliners kept at least 30 minute or more waits. At MK on our last day Space Mountain had a 65 minute wait at 3:30 p.m! I don't mean to sound like a downer, but I just wanted you to be aware that with Free Dining being offered the last few years, it has really increased the attendance levels at WDW. When we left we had every intention of booking the Free Diningbounceback for next September that they were offering, but my husband said he would happily pay the $1300 we saved on this trip and put it towards a slower, cooler time to visit.
That being said, if you can only go during September, you could pick the week of Labor day. Labor Day itself will be more crowded, but there were reports on the Dis that the rest of the week this year was pretty good as far as crowds. It will still be very hot that week though. The last week week may be your best bet. The temps will be cooling and that will make the crowds more manageable. The highlight of our trip was Mickey's Not So Scarcy Halloween party! It was awesome. Whatever week you pick, have a great time.