Our trips always start labor day weekend!
We have never done the "parks" that weekend, I always figured they would still be busy due to a) being a weekend, b) school getting ready to start etc... but we do do a waterpark on monday, labor day and they are not very crowded at all!
After reading that corkys mom did mk on that monday, and it was empty, I may rearrange our schedule! hmm.....
we usually do the parks the week after labor day, and you couldnt ask for better crowds! Very minimal wait times! (if any)
Weather.... it is hot and humid! Just like august. Not bad if you are used to it... if not used to hot weather, it may cool down a tad at the end of september, I am not sure!