21 May

21 May was the subject of a mystery that Sarah Jane Smith, the Third Doctor and Jeremy Fitzoliver had to uncover in 1975 Italy . 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 on BBC1, bringing to an end the regular practice of titling individual episodes until 2005.
 * 1974 - Actress Juliet Cowan was born.
 * 1985 - Actor Calvin Dean was born.
 * 1995 - Actor Peter Rutherford died.
 * 2005 - The Empty Child was first broadcast on BBC One.
 * 2011 - The Rebel Flesh was first broadcast on BBC One.
 * 2011 - Actor Bill Hunter died.
 * 2012 - The cast read-through for Hide took place.