Right now, it seems that the default UY being offered to new members is Feb. That does not mean you can not ask for a different one. Disney will work with you and get you whatever makes sense for you.
Your UY is when that years points become "available". If you know that you have travel times that will be pretty consistent over the next several years (and it sounds like you do), then you want to try for a UY that starts just before typical travel. While travel habits can and will change down the road, it certainly can be beneficial in the short term.
With May and November trips, the ideal UY would be April. That would give you to bank your points until November 30th. Any trips that were ever cancelled or changed at least 31 days in advance, would still leave you time to do something with those points (canceling or changing with less days then that set up a whole new set of rules for points).
In addition to banking, if you had to borrow points to cover those trips and had to cancel or change, you would have flexibility to use them up since once borrowed, they can't be returned.
I am a summer traveler and go mostly in August. We have a June UY which works out great for what I want to do. Since we make our reservations 11 months in advance, it allows me the flexibility to make changes. I can do that right up until my 31 days out and not have to worry about the leftover points, if any, from the changes.
Good luck!