I have booked several weeks at 13 months out via points this year and the calendar's of the resorts I'm interested in (Surf Club, Ko Olina, Crystal Shores) do not open up until 9AM EST on Tuesday's. Last week checking 8:59AM not available then refresh at 9AM and the week opens up. May not be all resorts but my experience using enrolled week points.
To clarify, this particular OP is questioning Weeks inventory/rules, but below is a basic rundown of both Weeks and Points.
The difference is strictly whether you are using Weeks or Points to book reservations and if it's Points, it doesn't matter whether they're purchased Trust Points or Exchange Points from Enrolled Weeks. The first step is to determine the Inventory Release Date for your particular desired usage/stay:
--> sign in to your owners.marriottvacationclub.com account
--> click on "Helpful Tools" under the "RESOURCES" tab
--> click on the "When Can I Reserve?" box
--> select either the Weeks or Points option (again, makes no difference if Points are purchased Trust Points or Exchange Points from Enrolled Weeks) and follow the prompts to input your desired resort/check-in date, after which the various release dates will be auto-populated.
After you've learned the release dates
using Weeks:
- the inventory is released at 9AM Eastern time and the high-demand inventory goes very quickly
- single Weeks can be booked using the 12-mos Reservation Window and the system supports online confirmation
- multiple consecutive/concurrent Weeks can be booked using the 13-mos Reservation Window and the system does not support online confirmation; high-demand inventory requires calling in to Owner Services at the moment of release
Or
using Points:
- the inventory is officially released at 9AM Eastern time but the online system may load very shortly after midnight on the Release Dates, and high-demand inventory goes very quickly
- the Reservation Windows are determined according to status tiers; see this
Benefits At A Glance chart
- all bookings are supported by the online system
If there's no availability at the opening of any Release Date, you should continue to search at any time after the window opens because other owners/members as well as Marriott may make changes any time that impact availability - inventory remains fluid until the moment of check-in. Note also that when using Points there's a Wait List option, but not with Weeks.