That's right, you need to be there early if you want to get a spot for the 12th.
I was able to attend last year, and the tent city filled up within hours of opening. If you plan to stay in a tent, arrive well before they allow campers, and bring ear plugs. You will not sleep without them.
If you are staying in a hotel, you can arrive on the 11th or even the morning of the 12th.