"The Day of the Doctor" is a special episode of the British science fiction television programme Doctor Who, marking its 50th anniversary. It was written by Steven Moffat, who served as an executive producer alongside Faith Penhale. It was shown on BBC One on 23 November 2013, in both 2D and 3D. The special was broadcast simultaneously in 94 countries, and was shown concurrently in 3D in some cinemas. It achieved the Guinness World Record for the largest ever simulcast of a TV drama and won the Radio Times Audience Award at the 2014 British Academy Television Awards. The episode was watched by 12.80 million viewers in the UK and received positive reviews from critics.

The 77-minute episode depicts the last day of the Time War, in which the War Doctor prepares to kill both Daleks and his own people, the Time Lords to end the destructive conflict, paralleling this with a present-day choice by paramilitary organisation UNIT to destroy London rather than allow an alien invasion. Revising the backstory, the Doctor succumbs to Clara Oswald's plea to change his mind; and instead he freezes his war-torn home planet in a single moment in time and hides it in a pocket universe, rather than destroy it; the time distortions incurred causes all but his latest incarnation to have no memory of the changed decision.

The episode starred Matt Smith as the Eleventh Doctor and Jenna Coleman as his companion, Clara Oswald. Previous lead actors David Tennant and Billie Piper returned for the episode; Tennant reprised his role as the Tenth Doctor, while Piper portrayed a sentient doomsday weapon called the Moment, projected as an image based on her character Rose Tyler. She is invisible and inaudible to everyone but the War Doctor (John Hurt). Other appearances included a brief glimpse of the then-upcoming Twelfth Doctor (Peter Capaldi), and a guest appearance by Fourth Doctor actor Tom Baker, as a mysterious curator. Rounding out the guest cast are Joanna Page as Queen Elizabeth I and Jemma Redgrave as Kate Stewart, the daughter of central figure Brigadier Lethbridge-Stewart. The special also featured the appearance of the Daleks and the return of the Zygons, shape-shifting aliens who had previously appeared only in Terror of the Zygons (1975).

As the episode celebrates 50 years of the programme, it references and alludes to various concepts featured throughout the show's run. It received critical acclaim and has been described by producer Marcus Wilson as a "love letter to the fans" and then-BBC One controller Danny Cohen as an "event drama".