23 April 1975: death of William Hartnell, who played the First Doctor from 1963 to 1966, and returned for The Three Doctors in 1972-73.
23 April 2013: death of Norman Jones, who played Khrisong in The Abominable Snowmen (Second Doctor, 1967), Major Baker in Doctor Who and the Silurians (Third Doctor, 1970) and Hieronymous in The Masque of Mandragora (Fourth Doctor, 1976).
23 April 2019: death of Edward Kelsey, who played the slave buyer in the story we now call The Romans (First Doctor, 1965), Resno in The Power of the Daleks (Second Doctor, 1966) and Edu in The Creature from the Pit (Fourth Doctor, 1979).
ii) broadcast anniversaries
23 April 1966: broadcast of "The Final Test", fourth episode of the story we now call The Celestial Toymaker. Dodo and Steven defeat Cyril the schoolboy at Tardis Hopscotch, and the Doctor solves the Trilogic Game; they escape.
23 April 2005: broadcast of World War Three. The Doctor, Rose and Mickey defeat the Slitheen by hacking into UNIT's computers.
23 April 2011: broadcast of The Impossible Astronaut, starting Series 6 of New Who. Amy, Rory, River Song and the Doctor receive a mysterious summons that takes them on an adventure to 21st century Utah and Florida in 1969. Along the way they meet Richard Nixon, president of the United States of America, and former FBI agent Canton Everett Delaware III.
23 April 2016: broadcast of Friend from the Future, introducing Bill Potts.