They are due on Jan 15th. However, if you arrange for direct debit from your bank account, it can be paid 1/12 each month. You can also pay monthly if you financed your purchase through Disney.
Just a note to say that we have been doing the dues by monthly auto debit since we joined 10 years ago, and it is a GREAT way to pay maintenance fees without feeling a big crunch just after the Holidays.