The following information was taken from the Wikipedia Website.
“The Time of the Doctor” is a forthcoming episode of the British science fiction television show Doctor Who, written by Steven Moffat and directed by Jamie Payne, which is due to be broadcast on Christmas Day 2013 at 7.30 PM on BBC One. It will feature the final regular appearance of Matt Smith as the Eleventh Doctor and the first regular appearance of Peter Capaldi as the Twelfth Doctor following his brief cameo in “The Day of the Doctor”. This episode is the first Christmas special to feature multiple monsters, including the Cybermen, Silence, Daleks, and Weeping Angels.
“The Time of the Doctor” is the third and final instalment in a loose trilogy of episodes, following “The Name of the Doctor” and “The Day of the Doctor,” which together serve as the Eleventh Doctor’s swan song. The episode addresses numerous plot threads developed over the course of Smith’s tenure, including the prophecy of the Silence and the Doctor’s fate on the planet Trenzalore. “The Time of the Doctor” is also the 800th individual episode of Doctor Who, the ninth Christmas special since the show’s 2005 revival, and Matt Smith’s fourth and final Christmas Special as the Eleventh Doctor.