Late April into May, September and end of November into December. Would a December UY benefit us?
Your travel plans span a lot of the year, so you probably wouldn't go wrong with any particular Use Year. This matters if you think that there's a risk of having to cancel vacations on short notice and if that's the case, I would choose a March UY or September UY if available. As you can see below, the difference is that with December UY you would have to cancel vacations at some of your preferred travel times a little earlier or have a little shorter period of time to use your points than with other UYs; it may never be an issue if you don't cancel vacations.
With a December UY, you would have to bank points by July 31, after which you could no longer bank points and would need to use the remaining points by December 1. So if you cancel Sept or Nov vacations after July 31, you could not bank your points and would have to use them before Dec 1. If you cancel your Dec or April/May vacations, you would have months to bank your points and all the way to Nov. 31 to use your points.
With a March UY, you would have to bank points by Oct. 31. If you cancel Nov.-Dec vacations after Oct. 31 you could not bank your points, but at least you would have until Feb. 28 to use them. If you cancel your April/May or Sept. vacations, you can bank points, or use them by February 28.
With a September UY, you would have to bank points by March 31. If you cancel your April-May vacations after March 31, you couldn't bank your points but you have until Aug. 31 to use them. If you cancel your Sept or Nov-Dec vacations, you can bank your points or you could use the points up to August 31.
I hope this helps.