At this time of year, people are oddly inclined to impose strict sanctions on themselves, in an effort to improve their health, wealth or moral character. There's a feeling that a new year provides an opportunity to start with a clean slate, as if the past never happened. At some point down the road, we'll be able to say "Hey, I've been smoke/alcohol/fat/whatever-free since '01!"
I've written before about anniversaries, and how they really don't mean anything. The year is simply the time it takes the earth to travel its orbit around the sun and, coincidentally, slightly more than 365 days. But ok, I'll grant that it's convenient to mark holidays and anniversaries around this, mainly because the weather's usually pretty consistent. How could we dream of a white Christmas if it sometimes pops up in August?
But if you're going to make resolutions at some fixed point in the year, why not make it your own personal anniversary ... your birthday? Other life milestones (drinking, driving, voting, AARP membership etc.) are tied to birthdays, so why not treat your personal goals the same way? Then you can tell those folks in the future "Hey, I've been self-indulgence free since I turned 30," or whatever.
This would help to spread the virtue around throughout the year. It would relieve the overcrowding at health club parking lots, but also lessen the January slump in sales of cookies and cakes.
My birthday is about halfway through the year, so this system really works for me. Each January, I decide to scrap my New Year's resolutions, deferring them to the much more logical birthday event. Six months later, I can just as convincingly justify waiting until the start of the new year.