Usual ressie rule: you can call 11 months (at home resort, 7 others) in advance of your intended CHECK-OUT date from WDW not your intended arrival date. Let's assume you want to go Dec 28, 2008, to Jan 2, 2009. The rule would thus mean you would not call until Feb 2 to book all days.
However, Christmas season can be very tough to get if you wait at all particularly for New Years Eve. So here is what you might consider, called a day-by-day booking:
Call Jan 29 and get the night of Dec 28 (Jan 29 is the earliest you can call for that), you reserve only that night and do so as if the check-out date is Dec 29 and thus exactly 11 months from that check-out date. Tell MS you are doing daily booking and will want the ressies linked.
Call Jan 30 and book night of Dec 29 and have it linked to first night. Call Jan 31 and get night of Dec 30; call Feb 1 and get night of Dec 31; call Feb 2 and get night of Jan 1, and then make sure at end of this process you the ressies are all linked. You end up with one reservation and a single confirmation number.
That kind of booking is seldom needed to assure getting a room but Christmas time and early December are two times to consider it.