When I was there, we were able to get tickets an hour before going in. Whatever website you go to lets you pick the time you want to go in
Advance reservation tickets may be ordered online at Recreation.gov (1.877.444.6777) 90-days in advance for a non-refundable reservation fee of $1.00 per ticket. All orders are final. Individuals may request up to six tickets. Groups may request a maximum of 55 tickets. Groups must reserve tickets through Recreation.gov. Advance reservation holders can retrieve tickets via their Recreation.gov account, confirmation email, or by downloading the Recreation.gov mobile app.
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