In my experience, September is very hot and humid. We went September 29 through October 7 last year, and it was in the upper 80s to low 90s every day, and very humid. It also rained nearly every day (sometimes for only a few minutes a day, sometimes on and off throughout the day).
I think people think, "It's not summer since it's September" and expect it to be cooler than it is. But September and October can be surprisingly hot.