You have to bank those points prior to the last four months of your UY. So with a Sept UY, you have to bank them before May 1. Once you hit May 1, you are too late.
Also, you can't book something outside of
DVC during those last four months to be used during those last four months.
Say you forgot to bank. There is a nice RCI/DVC exchange not too far from your home and it is available in August when you want to go. You can't book that in May, June, July or August to use in that same Aug.