The points you get in October of 2011 are good for trips October 1, 2011-September 30, 2012. If you are taking a trip this December, and you didn't use any banked or borrowed points, then they are coming from your 2011 use year.
However, if you borrowed points (this is a final transaction and you should *know* that you have borrowed or not - the CM will explain the rules of borrowing), then they came from your 2012 use year.
Points in your 2012 use year are good for trips from October 1, 2012 - September 30, 2013. You can borrow them but they become part of your 2011 use year; same expiration rules apply. You can also bank them if you don't need them, and they'll be a part of the 2013 use year.
You can check your member account online to see how many points are listed in each use year. If you have questions, call MS and have them show you where your points have gone. They'll let you know if you've banked or borrowed any points.