This years Christmas Special is entitled 'Twice Upon a Time' it was announced during the cast panel at the San Diego Comic Con.
Cast details so far: Pearl Mackie is returning as Bill Potts, and Mark Gatiss guest stars as 'The Captain' a World War One soldier, plus Peter Capaldi’s current Doctor teams up with the First Doctor, played by David Bradley.
Here's the trailer...
BBC America also posted a Peter Capaldi tribute, “The universe shows its true face when it asks for help. We show ours by how we respond.”