The above poster (with the most adorable cats!) is correct. BUT, I have heard some of the girls mention that they called like a year and a few weeks out so that they were, "On the books," for the date and time that they wanted.
You won't actually start talking to your event manager until after you sign and return the contract with your deposit. It's usually about the 8 month mark for that.
Good luck! It all happens so fast anyway!
xoxo