Fall amid spring are your best bets (October through December, March through May). In the spring the wildflowers are impressive. You can camp at the campgrounds for a small park fee (they have toilets and developed sites), and also off road for free!
I would say from late October to early may. and park doesn't get a lot of rain so is not common for flash flood. there is several campgrounds that cost money but also you can camp anywhere for free. make sure you are not destroying plants and you can camp. a lot of 4x4 trails and things to see. would recommend slot canyon, kids would love that.
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