20 October 1941: birth of Anneke Wills, who played Polly Wright, companion of the First and Second Doctors, from 1966 to 1967 and has reprised the role a couple of times for Big Finish in 2009-10.
20 October 2008: death of John Ringham who played Tlotoxl in The Aztecs (1964), Josiah Blake in The Smugglers (1966), and Robert Ashe in Colony in Space (1971).
20 October 2009: death of Hubert Rees who played the Chief Engineer in Fury from the Deep (1968), Captain Ransom in The War Games (1969), and Stevenson in The Seeds of Doom (1976).
ii) broadcast anniversaries
20 October 1979: broadcast of fourth episode of City of Death, the one with John Cleese, Eleanor Bron, and the punch that saves the universe. Really, if you haven't seen it, you ought to.
20 October 2008: broadcast of first episode of Secrets of the Stars (SJA). A mysterious astrologer is able to tell Sarah Jane's history with the Doctor; Clyde appears to be under his influence.
iii) date specified in canon
20 October 1901: The cargo ship Lankester is sailing from Madagascar to New Orleans with passengers including the Sixth Doctor, Peri and some even stranger entities - as told in the Big Finish audio Cryptobiosis (2005).