You are allowed to bank points only once, so yes they do expire. For example, if you get Feb UY 09 points and bank them, they would be good during your Feb 2010 UY (Feb 1st, 2010 until Jan. 31st, 2011).
You can make a reservation at your home resort 11 months before you travel. Your UY does not impact when you can make the reservation, just when you get your points each year. When you call MS to make a reservation, you do not have to have point at that time, but only at the time of travel.
If you travel in Sept or January, you do not want to have a Feb or March UY. You would probably want an August or Sept UY. You have the first 8 months of your UY to bank points and ideally, you want to travel during those 8 months so that if you have to cancel a trip more than 31 days in advance (less than that then different rules apply to the points), your points will go back into your account with no restrictions.