Use month/year has nothing to do with when you can stay or when you can call to make a reservation. You can stay anytime no matter what use year you have and you can always call 11 months prior to check out to make a reservation at your home resort (7 months from check out for a reservataion at a non-home resort).
Use month determines your banking deadlines. That is important only if you have to cancel a vacation that occurs towards the end of your use year.
For example, let's say you mostly visit WDW in October and have an October use year. That means your:
2006 use year points are good for stays bewteen 10/1/2006 and 9/30/2007.
2007 use year points are good for stays bewteen 10/1/2007 and 9/30/2008.
2008 use year points are good for stays bewteen 10/1/2008 and 9/30/2009.
etc.
If you have to cancel a vacation scheduled for October, 2006 (made only with 2006 use year points), as long as you cancel at least 31 days prior to arrival, you have until 9/30/2007 to reschedule your vacation or you may bank the points. If you bank, the points are now valid for stays between 10/1/2007 and 9/30/2008).
But what if you usually travel in July and have an October use year? Let's say your planning to vist in late July 2007 and need a whole year's worth of points to do it. That stay falls within your 2006 use year and so you book it with 2006 use year points. Let's also say that you find out in early June, 2007 that you can't go and have to cancel the late July 2007 vacation. You only have until 9/30/2007 to reuse the points and because of the banking deadlines, you may only bank 25% of your total annual allotment (25% deadline is end of June for an October use year). That means if you can't use the other 75% of your points by 9/30/2007, you would lose them.
If you regularly travel at the same time(s) of the year, choosing a specific use year makes sense (because you can decrease the chance of losing points due to a cancelled vacation). Many members' vacation habits change over the course of their membership, so what may be a "perfect" use year now may not be so perfect later on (and vice versa).
By banking and borrowing, you may use up to 300% of your annual allotment of points for a vacation.
Best wishes -
