23 April 1975: death of William Hartnell, who played the First Doctor from 1963 to 1966 and again in 1972.
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.