Do you mean Matt Smith’s first Christmas special?
No, one of the later ones. It might have even been his regeneration show… why that was the Christmas Special, I do not know. Series 5 was definitely the best of his era, but it’s impossible to approach the later seasons (and any of Capaldi’s) without being much more familiar with the show. It’s a lot like the original show in the 80s. And… well, both eras saw a huge drop in viewership.