Just to add to what Doc said, in the example he gave, if you needed to, when you call to make the reservation, you could even borrow points from your 2009 use year to make the reservation.
Example:
1. You own a 150 point June UY contract.
2. You have 100 points remaining in your 2007 UY
3. You have all 150 points available in your 2008 UY
4. You have all 150 points available in your 2009 UY
You want to make a reservation for Sep 2008 that requires 330 points. Sep 2008 is in your 2008 Use Year.
You may call this month (11-months ahead, assuming you're using home resort points) and make the reservation.
1. MS will bank your 100 2007UY points into your 2008UY
2. They will borrow 80 points from your 2009UY and put them into your 2008UY.
3. This gives you 330 points in your 2008 Use Year (100 banked, 150 current, 80 borrowed). Note: Current here means regular 2008UY points, because the reservation is in your 2008 Use Year.
4. They make the reservation.
Sometimes it's easier to see in chart form. A June UY chart would look like this:
Use Year.....Vacation Dates
2007.....Jun 1, 2007 thru May 31, 2008
2008.....Jun 1, 2008 thru May 31, 2009
2009.....Jun 1, 2009 thru May 31, 2010
2010.....Jun 1, 2010 thru May 31, 2011
A September 2008 vacation falls within a June 2008UY. You can bank points from one use year to the next (EG bank 2007 into 2008). You can borrow from one UY to the prior UY (EG borrow 2009 into 2008)
You make the reservation at 11/7 months ahead of the checkout date, regardless of what your use year month is. Thus in the above example, someone with a December UY month would need 2007UY points for a September 2008 reservation. They could also call today and make that reservation, and if needed, use banked 2006 points, current 2007 points, and borrowed 2008 points. For them, current 2007 points are points they will be receiving Dec 1, 2007. Today (October) they are still in their 2006 Use Year.
Hope this helps.
Love the screen name! 
