Probably not best Use Year month you could have if you are rather certain that your travel will be in September for years to come and that would be because of the following reasons :
December Use Year has a banking deadline of July 31.
Example for your situation : Your trip is for Sept. 15 2009 and you are utilizing your December 2008 Use Year points to make this trip. These December 2008 UY points have a banking deadline
of JULY 31, 2009.
If for some reason you need to cancel your vacation between August 1, 2009 and before Sept. 14 2009 you would NOT be able to bank those points into your December 2009 Use Year which means you would need to use them or lose them by November 30, 2009.
Also, it is important to note that if you cancel a reservation less than 31 days before your check in date those points will go into "Holding" status and can't be banked and must be used by the end of your Use Year even if you haven't reached your banking deadline. So using the example above of a trip beginning on Sept 15, 2009 : if you cancelled this trip on let's say August 17, 2009 then those December 2008 UY points would NOT be banking eligible because they would go into "Holding" and you would need to utilize them by November 30, 2009.
You're travel habits may change as time goes on but if you are pretty certain you would travel in September most often than not then a June Use Year month is probably better -
so for Sept. 15 2009 trip you'd be using your June 2009 UY points and if you cancelled your Sept. 15 2009 trip more than 30 days before check-in you would still be able to bank those points & would have until January 31, 2010 to do so.