21 December 1915: birth of James Cairncross, who played Lemaitre/Stirling in The Reign of Terror (First Doctor, 1964) and Beta in The Krotons (Second Doctor, 1968-69). (he's also the parson in Tom Jones.)
21 December 1937: birth of Sheila Reid, who played Etta in Vengeance on Varos (Sixth Doctor, 1985) and Clara's grandmother in The Time of the Doctor (Eleventh Doctor, 2013) and Dark Water (Twelfth Doctor, 2014).
21 December 1991: death of Colin Douglas, who played security chief Donald Bruce in The Enemy of the World (Second Doctor, 1967) and lighthouse keeper Reuben in Horror of Fang Rock (Fourth Doctor, 1977).
21 December 1998: death of Roger Avon, who played Saphadin in The Crusade (First Doctor, 1965), Daxtar in The Daleks' Master Plan (First Doctor, 1965) and Wells in Daleks - Invasion Earth 2150 AD (Cushing movie, 1966)
ii) broadcast and stage anniversaries
21 December 1963: broadcast of "The Dead Planet", first episode of the story we now call The Daleks. The Doctor, Ian, Susan and Barbara land on a strange world with a petrified forest and an abandoned city. But what is it that terrifies Barbara at the end?????
21 December 1965: first performance of the stage play Curse of the Daleks, by David Whitaker and Terry Nation.
21 December 1968: broadcast of eighth episode of The Invasion. Tobias Vaughn changes sides and helps defeat the Cybermen, though he too is killed.
21 December 1988: broadcast of second episode of The Greatest Show in the Galaxy. Ace is captured by the clowns; the Doctor is forced to perform for the circus.