No matter what you do if your stay is from December 26th to January 3, it will post for credit in 2013
The only way around it is to have one reservation from December 26 to 30 (checking out on the 31st), go to another hotel for at least one night, and finish off the stay.
Checking in and out, if at the same hotel, will all be combined into one stay and will credit on January 3.