I would recommend arriving early. They open at 11, so I would plan on getting there by 10:30. If you hit the more popular rides first you can have a really productive day. My personal feeling is that if Great Adventure is open and the weather is forecasted to be nice, then it will be crowded.
I just went Saturday Sept. 16th and it wasn't crowded I think I waited no more than 30minutes for kingda ka and 40 for the safari which is honestly pretty face for six flags
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