If you're going to book a package (room, tickets, dining) you'll need to wait until the next year's rates are made public. The 2014 package rates were released in July so if you're looking to travel then you can now make a reservation at any point in time. If you're looking to travel in 2015, you'll need to wait until the rates are released in the summer of 2014.
If you're looking to book just a room you can do so 16 months in advance. I believe the "technical" number is 499 days, which equates to a bit more than 16 months, but I could be wrong.