Their policy is that they never turn people away. Last year they sold out online about a week before the festival and shit hit the fan. People were freaking out left and right, and they announced the above policy and said you could buy a pass when you arrive. Sell out or no, they've always sold tickets at the gate. You'll be just fine!
Oh, and the campground is a massive, circular field. There is plenty of room for everyone no matter when you show up. Granted, it'll be harder to find a spacious site anywhere near the festival entrance Saturday evening, but you'll find something.