You can buy a one-day pass at any national park at the front gate or you can buy a one-year pass at the front gate. as mentioned in another post one of the big issues with Zion right now is the shuttle passes you have to get online get in the lottery to get a shuttle pass if you cannot you will have to walk the entire way
Yes, you can buy passes on the day of arrival. What you need to worry about are shuttle passes. You can't use your vehicle to get to the trails. Go on to Zion's website to see what shuttle pick up times are available.
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