As drusba points out, its a misnomer to label this deed as a "guaranteed Christmas week." Based on the description on the Timeshare Store's website, the deed is for Fixed Week #51. In 2014, Fixed Week #51 owners check in on December 21 and check out on December 28. In 2016, Fixed Week #51 owners will check in on December 18 and check out on Christmas Day, December 25.
But in 2017, Fixed Week #51 will miss Christmas Day completely. Fixed Week #51 owners will check in on December 17 and check out on December 24.
I'm somewhat surprised that TTS is advertising this deed as a "guaranteed Christmas" week. I realize that a buyer is responsible for understanding what they are buying but, in my opinion, TTS should do a bit better job of describing what this deed really offers.