There were no discounts last year for arrivals 12/26 - 12/31. Even last year, there was no need for Disney to do this.
If you are planning to go during this time this year, I'd plan on making two separate reservations. For the first ressie, you'll have to pay rack rate (for 12/29 - 12/31). Then for your second ressie, you can pay value season rates for the remainder of your stay and perhaps even get a discount. Good luck!