I've only heard that Halloween sells out for sure, but I always think it's best to buy the tix ahead of time. Also, make sure you check the dates on the back of the tix when you get them. They sent us the wrong ones and I never checked until like the week before we left. Instead of making a big deal of it, I just juggled some things around and we went on the date they gave us. I figure that'll give me good karma for the next time I need something to go right.
Have fun!