UY month tells you 2 things:
1) when you banking window falls (8 months after the start of your UY), and
2) when your pts are valid for stays.
UY has
nothing to do with when you can call to book.
if you never cancel, UY month will never be an issue. if you need to cancel, you will have more options to work things out if you are traveling early in your UY.
ideally, you'd like to travel in the first few months of your UY and
never travel in the last few months of your UY.
for primarily sept-nov trips, a sept UY is probably the best choice (assuming you'd never/rarely travel in june-july-aug).
for more info, read this:
http://www.disboards.com/showthread.php?t=1942668