You really have all of your points already. They don't deposit into your account at the start of your UY, but simply become valid for stays on that date.
If you look at your
DVC member account, you will see points for several UY's ahead of it.
OP, as mentioned, everything is based on the points that will be available for travel at the time of the trip and not when you call.
So, with October travel every year, you will always be able to call in November to set that up.
MS will look at your account and use the points eligible for that trip to book it. So, if you decided to get a Sept UY, and were traveling in Oct 2012, you would use banked 2011 UY points, 2012 UY points, and borrow 2013 UY points for that trip, since it falls in the Sept 2012 UY.
However, if you had a Dec UY--you could still call 11 months out--but would only be able to use banked 2010 UY points, 2011 UY points, and borrow 2012 UY points because an Oct 2012 trip falls in the Dec 2011 UY.
What you are really looking for is a UY to begin just shortly before your preferred time of travel. Since you already know that will be Oct, go with Oct.
However, it can also be important to think about other times of travel--are you more likely to travel in the winter or summer if you didn't go in Oct. If it is summer, then an Oct UY isn't very good and you might want to look for April or June as it would cover both summer and fall travel.