Well, booking is available up to 500 days in advance. From today 07/19/16, 500 days out is December 1, 2017. I played with the reservation tool and the "500 day" mark seems to apply to the last night of your reservation. So today if you leaving on December 2, 2017 (meaning your last night at the Fort is December 1), you will get prices and availability. Anything beyond December 2 checkout (as of today) will come up empty.
I use this tool
http://www.timeanddate.com/date/dateadd.html to calculate my dates for ADR's etc. So determine your last night on your vacation, subtract 500 days from it, and it will tell you when you can make the reservation (in theory).
bgula, seems like you sure do like Christmas at WDW looking at your signature. Enjoy!
Bama Ed
PS - or try calling around that D-day.