I quit smoking easily last year on January 1st. I decided to quit a month prior to Christmas, but I was afraid of the jolliness of the season making me slip. I assume it is the same for exercise, portion control, and alcohol consumption moderation.
Sometimes it is nice to have a clean break in a month where nobody bothers you and where you haven't had a routine for a couple of weeks.
Although I really wish the gym wouldn't pack up so much.