21 May (releases)


 * 1966 - "The O.K. Corral" was broadcast on BBC1, bringing to an end the regular practice of titling individual episodes until 2005.
 * 2005 - The Empty Child was first broadcast on BBC One. Later, Special Effects aired on BBC Three.
 * 2011 - The Rebel Flesh was first broadcast on BBC One. Later, Double Trouble aired on BBC Three.