Christmas Day? Nice. Then again The Sun isn't exactly reliable.
I was thinking about the timeline of Series 3 the other day, and it's actually quite jumpy. The Empty Hearse takes place two years after The Reichenbach Fall which was presumably set in 2012. However the former also takes place on November 5th, Guy Fawkes night. So November 2014. Then doesn't Mary say that her's and John's wedding is being planned for May, which would make it May 2015?
Cut to His Last Vow, with the last half hour/45 minutes set on Christmas Day with John not having spoken to Mary for a few months. The last ten minutes is presumably set a couple of weeks later, making it sometime in January 2016.
That's an extremely crazy timeline. And January 2016 marks a likely airdate for Series 4...