If you're up for a bit of a read/tutorial, here's some help:
Understanding Use Year -
Use year is the month your points are renewed each year. You have 12 months (one year) to use the points before they expire (unless you bank them).
Use year determines your banking deadlines, but it has nothing to do with when you can make a reservation. You can always call 11 months prior to check out for a reservation at your home resort and 7 months prior to check out if you want to reserve something at a non-home resort.
Let's say you get October for your use year. Let's say you decide to buy 200 points.
On October 1, 2008 you get 200 points to use for stays between 10/1/2008 and 9/30/2009. Those would be considered your current use year points for the 2008 use year and any stays that occur during that time period. (If you buy now from Disney and get an October use year, you also get 2007 use year points with your purchase).
If you don't plan to take a trip before 9/30/2009, you'll want to bank your 2008 points so that they do not expire. As long as you call before the end of May 2009 (the banking deadline for 2008 points with October use years), you can bank all 200 points. If you have an October use year, you cannot bank at all during June, July, August or September. (The banking deadline is the end of the 8th month following the start of your use year month. For example, if you have a June use year, the deadline would be the end of the following January. You would not be allowed to bank during February, March, April or May).
Your 2007 use year points are good for stays between 10/1/2007 and 9/30/2008.
Your 2008 use year points are good for stays between 10/1/2008 and 9/30/2009.
Your 2009 use year points are good for stays between 10/1/2009 and 9/30/2010.
Your 2010 use year points are good for stays between 10/1/2010 and 9/30/2011.
etc.
You may bank points forward one use year and borrow from one use year ahead. Banking and borrowing are final transactions. If you cancel a reservation made with banked or borrowed points, they cannot be returned to their original use year. So they would expire at the end of the current use year unless they were used before then. You may not bank or borrow the same points more than once.
Let's say you want to stay in May of 2009 and your stay will cost 500 points. May, 2009 falls within your 2008 use year (See the "chart" above). For that reservation, you may use current use year points from the 2008 use year, any banked points originally from your 2007 use year and you may borrow points from your 2009 use year.
Let's assume you banked all 200 of the 2007 use year points. So for the May 2009 vacation, you used all of the banked points, all of the current use year points and borrowed 100 points from your 2009 use year.
Now let's say you have to cancel that May, 2009 vacation. You call to cancel it in March, 2009. Since you called to cancel more than 30 days prior to arrival, all 500 points were returned to your account. But now you have 300 points to use or lose before the end of your current use year (9/30/2008) - the 200 that were banked and the 100 that were borrowed.
If you called Member Services before the end of May, 2009, you could bank all 200 of your 2008 use year points if yo didn't think you would use them before 9/30/2009.. Now those 200 points are good for stays between 10/1/2009 and 9/30/20010. They will expire on 9/30/2010 if not used before then - you cannot bank the same points more than once.
ADVICE: If you travel about the same time of the year on a regular basis, it's best to choose a use year that falls no more than 6 or 7 months prior to that time.
Use year only matters if you have to cancel a vacation that was scheduled to occur late in your use year - because banking is limited late in the use year and you wouldn't have much time to reschedule before the points expire).
For example, let's say you wanted to take a 500 point vacation in August of 2009. Again, you use 200 banked points, 200 current use year points and 100 borrowed points for the reservation. Let's say you find out in mid-June that you can't go and call to cancel the reservation. Your account now has 500 points that expire on 9/30/2009 if not used. Since you are past your banking deadline, you can't bank any of the 2008 points (like you could when you were planning to go in May and had to cancel).
Many owners never have to cancel a vacation (lucky them, LOL) so they don't worry about use year. Remember, use year has nothing to do with when you may call to make a reservation. Again, if you usually travel at the same time of the year, it's best to choose a use year that begins just prior to that time.
Keep in mind that your travel pattern may change over the course of your ownership. What starts out as a "perfect" use year may end up to be not so perfect.
Hope all that helps you understand a little better how use year works.
Best wishes -
So, does use year depend on when you actually buy into DVC? Is that something you even need to consider? TIA!
If my family normally vacations in May or early June, what would the best UY be? It makes sense to me that if, for example, I were to buy a September UY, make a reservation for May 26, 2009, and need to cancel 30 days out, I'd still be able to bank the points since April is the 8th month. Is that wrong? Am I overlooking something? 
