The Aztec civil calendar consisted of 18 "months" of 20 days each, for a total of 360 days, followed by 5 days called the nemontemi (neh-mohn-TEH-mee), or "useless days". These days were considered unlucky. People would lie low during the nemontemi, delaying the start of new projects, keeping to their homes as much as possible, until the new year began.
The week between Christmas and New Years has no official name in our culture; I can think of few better names for it than "the Useless Days". We certainly treat it in much the same way as did the Aztecs. So, happy (if quiet) nemontemi!