Breaking your resolutions

It’s the first week of January which means it is the traditional time to break all the resolutions that we all set a few days ago.

I am always confused why people determine that they will change their lives based upon the arbitrary date chosen to start a new calendar. Is there a difference to your willpower from the 31st December to the 1st January, what is it about the changing of the year that makes people decide that this is now the point at which they are going to the gym or to sort out their lives?