But then, who is going to accept your money? If you're the local baker, why would you take my money for the bread you made, knowing perfectly well that you're probably never going to get the chance to spend it?
Doing business on Dec 30 or even Dec 31 is the extreme case, but even before that it's an issue: the closer you get to the expiration date of the money, the higher the risk you're taking that you might not be able to spend it in time.