Thanks everyone for all the informative replies. I was considering going to a state park this weekend, but I didn't have reservations.
Here is the situation: It is friday night, and I want to camp out sat and sun night at Half Moon Bay State Park. I go to online reservations: I can book sunday, not saturday (because 2 day advance is required). I call the park, I get recordings that say book online. So, if I drive down there on Sat, and they have sites available, is there someone there that will let me book one? Or do i have to book online only?
I had the same problem with a couple of county parks. One wanted a 5 day advance reservation. So, I decided to go to a private park in Cloverdale, CA instead, and it was great (I live near San Francisco). We had company flying in from Chicago that evening, and I didn't want to chance not getting a site. The park was mostly empty, and the owner even helped me back my trailer into a site (a first for me).
Now, a couple of you mentioned cancellations. How do you find out about those? Can you call the park? I haven't had any luck with that on weekends/holidays, as no one answers.