Interest you pay on a home mortgage is tax deductible – for the year in which it was paid. By making January's payment in late December, you not only get a little ahead, but you get to deduct the mortgage interest you pay in this tax year, not next year. Say sending an early payment results in an additional $100 or $200 on your income tax refund; in effect you get that money a year earlier than you would have by waiting a couple weeks to send in your mortgage payment.

Just make sure you send the payment in with the proper payment slip or otherwise make sure the lender knows you're making a payment ahead of time instead of sending in extra money that should go to paying down your principal balance. Some lenders assume additional – or early – payments should go to paying down principal, not to making an early payment.