We did it last year, and I booked right at the 90-day mark. However, since then "Priority Seating" has changed to "Advanced Dining" and the advance reservations period has increased to 180 days.
We went on the very first day of the processional, which was the Friday after Thanksgiving. However, Thanksgiving was late last year, and is very early this year (November 23), so it may not start as soon this year as last year. As of today, there's nothing on the WDW website about it.
The price, as I recall, was about $40 (a bit less) for one tier or restaurants (that include Chefs de France and Alfred), like $30 for tier two (Rose & Crown?) and $20 or so for tier three (San Angel Inn).
So, nothing definitive, but hope this helps at least a little.