21 May

21 May was the subject of a mystery that Sarah Jane Smith, the Third Doctor and Jeremy Fitzoliver had to uncover in Italy one year in the late 20th century. They kept encountering references to the day on their holiday to the Brig's ancestral home. Eventually, the Doctor discovered that it was the date of the return of Clancy's comet. The return amplified the ability of people to cross over into N-Space. (AUDIO: The Ghosts of N-Space)

Behind the scenes

 * 1932 - Actor Jay Neill was born.
 * 1953 - Actor Trevor Cooper was born.
 * 1966 - "The O.K. Corral" was broadcast, bringing to an end the regular practise of titling individual episodes until 2005.
 * 1995 - Actor Peter Rutherford died.
 * 2005 - TV: The Empty Child was first broadcast.
 * 2011 - TV: The Rebel Flesh was first broadcast.
 * 2011 - Actor Bill Hunter died.