20 November 1930: birth of Bernard Horsfall, who played Gulliver in The Mind Robber (Second Doctor, 1968), a Time Lord in The War Games (Second Doctor, 1969), Taron in Planet of the Daleks (Third Doctor, 1973) and Chancellor Goth in The Deadly Assassin (Fourth Doctor, 1976).
20 November 1984: death of Peter Welch, who played Sergeant Klegg in The Highlanders (Second Doctor, 1966-67) and Morgan the pub landlord in The Android Invasion (Fourth Doctor, 1975).
20 November 1994: death of John Lucarotti, writer of the stories we now call Marco Polo (First Doctor, 1964), The Aztecs (First Doctor, 1964) and The Massacre (First Doctor, 1966).
20 November 2000: death of Morris Barry, who directed The Moonbase (Second Doctor, 1966), Tomb of the Cybermen (also Second Doctor, 1966) and The Dominators (Second Doctor, 1967), and then appeared as Tollund in The Creature from the Pit (Fourth Doctor, 1980).
ii) broadcast anniversaries
20 November 1965: broadcast of "Day of Armageddon", the second episode of The Daleks' Master Plan. The Doctor infiltrates the Daleks' meeting in disguise, and seizes the terranium core from Mavic Chen. I just watched this again the other day, and it's really good! (Though what happens to Mavic Chen's crew???)
20 November 1976: broadcast of fourth episode of The Deadly Assassin. The Master fakes his death and attempts to unleash the forces of the Eye of Harmony. The Doctor defeats him, though with much devastation to the Capitol.
20 November 2003: webcast of second episode of Scream of the Shalka. The Doctor blows up Alison's cafe and her house to stop the aliens, but then gets taken up by the army; meanwhile the Master [played by Derek Jacobi] has got into the Tardis.
20 November 2009: broadcast of second episode of The Gift, ending the third season of the Sarah Jane Adventures. K9 persuades the Rakweed to explode, killing off the Blathereen / Slitheen.
20 November 2013: broadast of The Last Day, another of the 50th anniversary prequel minisodes.
iii) date specified in-universe
20 November 1987: Energize!!! (Seen in Father's Day, Ninth Doctor, 2006)