Now that it's open year-round you can go anytime in the spring or fall or winter. I suggest planning a trip around Halloween because Halloween nights are really fun.
You can always cool off on one of three water rides, or you can stay hydrated. Anytime of the year besides the summer is a good idea if you're looking to avoid the heat entirely, but make sure to check the weather and especially the UV intensity. If you have a flash pass, heat is no problem, really. I'd suggest early spring and late fall for a good balanced climate, but always know what you're getting into beforehand!
Now...good weather. And better to go before schools are out
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