Everything is based on when you travel and not when you book. Since a May 2024 trip is in your December 2023 UY, you can borrow 2024 points for that trip, no matter when you go to book it!
Rule to remember: to determine which points are “current” use year points for a reservation, whether you need to bank points, and whether you can borrow points, wipe from your mind the date you go online or contact MS to make the reservation, and look only at the dates you will be at the DVC resort.