The sites in the state parks are reservable, so you could probably reserve them now if you wished. IIRC, in the parking lot near each bank of sites, there is a drop box/reservation board that tells you of what sites are open. I'd imagine if you're hiking through the area on the weekend, it would be well advised to reserve something ahead of time. One year we got to the state park and all sites were occupied or reserved and we had to (illegally) camp out in a clearing near a park service garage.