Free dining is usually offered earlier in the Fall (think September). We visited Thanksgiving 2014 and the parks were crowded until Sunday. If you look at the individual days of your resort reservation during a discounted period, many times Friday and Saturday are not discounted days.
But if you want to go for Christmas, either do early (week or two before Thanksgiving) November or the first week or two after Thanksgiving. Either your first or last travel dates would be fine.
Make your resort reservation as soon as you firm the dates. As other posters said, once discounts for late Fall are released, which should be soon, you can call Disney travel to adjust your existing reservation. Just note that they may not have discounted rooms available at the resort you previously selected. I think the idea of using a travel agent is an excellent suggestion, since they will track the discounts and make the adjustment for you. I would also recommend Kingdom Konsultants as a travel agent... they are a sponsor of this forum.
You can also go to Mousesavers.com and see when late Fall discounts were released last year, along with what the discount was for each resort level and what resorts were included.
But don't wait too long....
Edit: re-read your original post and realized you were talking about 2017. Not sure if you can make a resort reservation this far out, since Disney may not have put rates in their system yet. Bur doesn't hurt to check.