If you need to bank by the end of September, then it sounds like you have a February UY.
A May 2014 trip will take place in your Feb 2014 UY, so if you want to use the 2013 UY points, then yes, you will need to bank them before making the reservation.
Once they are banked, they will use those first and finish it up with any 2014 UY points you need as well as borrow from 2015 UY if you didn't have enough between the banked 2013 and the 2014.
Everything is based on when you travel so when figuring out what points can be used for a trip, just figure out what UY your trip will happen and then you know that you can "pay" for it with banked points from the UY prior to travel, points from the UY of travel and borrowed points from the UY after travel.
Of course, if you decide to book with MS via phone, make sure that they use your banked points first. They should but its always good to double check. Doing it online is easy and you can see what is being used when you make the reservation.
Good luck!