It depends...in September, most parks are great to go to because there is no one there. However, many parks, including all the Six Flags parks (at least on the east coast) have some sort of Halloween celebration in October. These draw the crowds back to the parks before they close for the season. It if fun to go to these events, but it is extremely crowded then and if you just want to get on the coasters, it makes it hard to do.
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--Chewman
http://greatadventure.8m.com