Rule to remember: you can always book 11 months out from arrival date at home resort and 7 months out for others. To determine which use year points you can use when doing so, and whether you need to bank or borrow, wipe from your mind the date you are going online or calling to make the reservation, and look only to the dates you will be at WDW.
Example: if you have a Dec use year, you could today, Oct 21, 2020, go online and book a vacation starting September 21, 2021, and in doing so you could use Dec 2020 use year points as your “current” use year points for that trip, any Dec 2019 points that have been banked into the Dec 2020 use year, and, if needed borrow points from the Dec 2021 use year.