Much like stuffing Christmas stockings with candy, or figuring out what ugly sweater to wear to the office holiday party, the Doctor Who Christmas Special is a time honored tradition.
During the events of that special, a character by the name of Nardole was introduced, and shown to be a sort of companion to River herself.
Since the show’s revival in 2005, a yearly offering of The Doctor saving the world at Christmas has been upheld, with “The Husbands of River Song” being the most recent offering in the line.
At least, that’s what the tweet below seems to say.