Half of the sites are reserved, half are first come first served (FF). If looking at the campground map, Lilac, Pinyon, Yucca, and Amphitheater are reservable. If spots on those loops have not been booked within 4 days (I believe) they become FF sites. So it is possible you can camp in one of those spots last minute, if they have not been reserved.
All, I think, are booked in advance.
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