I would like to travel in either May, September or December. What would be the best use year to purchase?
If I purchase an August UY, what is the timeframe for making ressies?
Your UY comes into play if you ever have to cancel a trip. Ideally, you want a UY where you normally travel early in your UY or at least within your banking window(s). In order for your points to be bankable, you must cancel at least 31 days prior to check in, otherwise your points go into the "holding account". Points in holding cannot be banked and can only be used to make a reservation 60 days or less in advance of check-in. If you cancel on your check-in day, all points are forfeited.
So in your case, a Sep or an Apr UY would work out very well. Either would cover all of the travel dates you listed. Under the new banking rules going into effect in 2008 your 100% banking window for a Sep UY would be from Sep 1 - Apr 30th, and for an Apr UY would be Apr 1 - Nov 30. Since you would have to cancel a May trip sometime in April (and a Dec trip in Nov) to avoid the points going into holding, you would still be within your 100% banking window.
The deciding factor between Sep or Apr would be whether you might travel in the summer as well, in which case you should go with an Apr UY, or if, like me, you will NEVER go to WDW in the summer in which case a Sep UY would be better.
Be aware that as we're seeing now, banking rules can change at any time so what might be the perfect UY today could present problems in the future, but again, only if you have to cancel a reservation. You can book a stay at your home resort 11 months prior to check-out, 7 months prior at non-home resorts, regardless of your UY.