We've been there as early as late September and as the last week of October. October was much more crowded often to the point of it seeming like a normal day. September has maybe moderate to light, have done it during that month twice. The downside of doing it earlier is that the weather will be a little hotter than October by 4 or 5 degrees so you have to be more careful in your costumes. The upside was that because it was very sparse crowd wise in the September visits the candy stations were handing out a lot more stuff, pretty much dumping it by the end of the night, I recall on of those years we had to get a cheap carry-on bag to get our daughters candy back home because it wouldn't all fit in the suitcase.
If you are wanting less crowds then do the earlier parties... I assume they have the same nights as usual so avoid Friday nights because they tend to be more crowded than Tuesday or Sunday nights.
