A month is a much better granularity than the more typical yearly resolution we make on new years, because you can estimate better on that smaller scale, and recover and reset faster. And it's long enough (over 21 days!) for some habituation to occur, even with a slip up or two.
Limit your celebratory eating to the holidays themselves. Adjust your expectations so that your goal is mere maintenance. If you screw up, don't wait till new years to get back on the wagon.
One thing that every self help guru can agree on is that you have to have an arbitrary number in your system. It is a little weird that the most rational concepts we have available to us -- numbers -- are so irrationally inspiring. But it clearly is the case.
Need to keep track of something? Keep track of behavior, not results, with the Habit Traffic Light. Mark each calendar day with green for success, red for failure, yellow for exempt (S-days). It's cheap and keeps you focused on what you can control.