I've been three times in Sept...2nd, 3rd and 4th week. Crowds during all three weeks were extremely manageable, but the second week was the most sparse. The weather is usually very hot and humid (atleast for this Canadian!)...most days in low 90s with humidity. Usually rained for 30 minutes in late afternoon, but didn't distract us. Since we found it hot, we usually started out first thing in the morning at the parks and then went back to hotel for rest by 1:30 and went back out around 4:30 to finish off parks or have dinner. I think Sept is a great time for low crowds, but be prepared for hot days with short bursts of rain.