are predermined (like sites 2, 9, 30...), if they're taken, then no more. I always got one at Algonquin, Kingston, Kill Bear, etc. (always 1 day ahead). The first time I got to Algonquin before 11am. Later times I found that there's no problem as late as 1:30pm(but the earlier, the better, you still have chance to choose a better site). Choose a bigger site to try(so they have more sites).