Sept UY would be the best. YOu want a UY that starts before you typical travel to allow for the most flexibility if you have to cancel a trip, are using borrowed points, or simply want to adjust your trip and it uses less points.
In addition to Sept, you should also think about when else you might like to travel as that can impact UY as well.
We went with June UY, because our typical travel is summer (I teach). However, once I retire (9 1/2 years!!!!), we will most likely start going in the fall and/or winter. Knowing this, we realized that June works well for those times as well.
With a Sept UY, traveling in summer (June, July, August) becomes tricky as points expire August 31st. If you ever had to cancel a trip, or changed one, after your banking window of April 30th, you wouldn't have much time to use up the points.
If you think that summer could be an option, then go with June UY. That covers trips all the way through February and leave the spring as the most vulnerable.