If it is a floating week, you can reserve any week within your season that is available starting at 12 mos. out..
If it is a fixed week, you can only reserve that week. However, you can try to exchange it for another week in the Starwood system by requesting a Staroption exchange at 8 mos. out. It is difficult to reserve a different week during high season at the most popular resorts, especially Harborside and Westin St. John. Not all resales are eligible for Staroption exchanges.
For lots of info. about how Starwood works see the FAQ at the top of the board.