No there isn't anything special other than you're their on "real halloween"
As for crowds:
I went 4 times and worked one at least once a week this past fall. I went to a couple in September and a couple near Halloween. The early September was crazy hot this past year O.O The Mid/End September one was just the right amount of crowds and not too hot to wear a costume. Beginning of October was pretty decent too and the last party November 1st, which seems to be the backup for people that can't go to the 31st, was really crowded. So, personally, I would shoot for Mid September-Mid October for a smaller crowd, but that's just my opinion.
If you want to go to the 31st, make sure to get your tixs early and have fun ^_^