To give an exact answer you would need to post your actual travel dates in December.
Bookings are always 11 months from the CHECKOUT date. If you want to stay 6 nights, checking in on 12/1 and departing on 12/7, you would call on January 7 to make the reservation.
Or, if you wish, you can call and book each individual date one at a time. In the above example, you would call on January 2 (or the first business day following) to book the night of 12/1, and then you would continue to call each subsequent day until the entire trip was booked.