Not really, but there's an advantage to having one particular day on which to do things, to avoid the "I'll do it someday" problem.

I think that's probably true, although I think it's along the lines of what I mentioned earlier -- a kind of internal Schelling Point.

There's also the problem that it might create another kind of "I'll do it someday" problem. For example, a person might pig out on Thanksgiving and Christmas and justify it to himself saying "Starting January 1 I will eat healthy and hit the gym every day."