To understand points and when they can be used, you base it on travel and what UY that happens.
So, if you buy a contract with a Dec UY and want to travel in Nov 2011, that will trip will be in your 2010 UY (Dec. 1st, 2010 - Nov. 30th, 2011).
Trips can always be paid for using banked points from the previous UY, the current UY of travel, and borrowed points from the UY after.
In your case, you could borrow the 2011 UY points to pay for the Nov trip. The drawback with this, however, is that points borrowed can not be returned and if you had to cancel, you would lose those points as they would epire Nov 30th, 2011.
If Nov is your travel is typical, I would not consider getting a Dec UY contract. No matter when you travel, you can book 11 months in advance so you don't need a Dec UY to book Nov travel (just in case you thought this).
Good luck!