I agree with most that there is usually same-day availability. However, we went on Christmas. I tried to move up our Star Wars reservation to an earlier time. They were completely booked. If you're considering going on a major holiday, you might want to account for this.
I'll also second the person who said that the front-of-house staff are a bit unorganized. That's a small price to pay for an excellent experience. We've now done the Star Wars experience 3x and Wreck It Ralph 1x. And I imagine that we'll do both again the next time we visit.