10 am
I went about 2 weeks ago and it was very crowded on Saturday and Sunday. We had to get the fast passes just to make sure that we were abke to go on most of the rides. I would recommend going during the week, it would most likely be less crowded. Also Parking is $15.
As far as what time of the year to go, I recommend going after kids are back in school. We went a week ago and it was just swarms of people! However, the lines for the roller coasters seem to go quickly but I recommend the fast pass if you plan on going on them. I believe you can only use it once per ride though, and it's only good for 12 or 13 rides. If you're asking about time wise, I say get there between 9-9:30am to get good parking.
10 am need more then 1 day if you really want to enjoy
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