24 April 2008: death of Tristram Cary, who wrote incidental music for six First Doctor stories and two later ones.
ii) broadcast anniversaries
24 April 1965: broadcast of "The Space Museum", first episode of the story we now call The Space Museum. The Tardis jumps a time-track and the crew find themselves in a museum where they cannot interact with the locals and they themselves appear on display.
24 April 1971: broadcast of third episode of Colony in Space. The Doctor and the colonists take control of the IMC ship, but Jo is in the hands of the primitives.
24 April 2010: broadcast of The Time Of Angels. The Doctor, Amy, and River Song find themselves trapped on the Byzantium with armed Clerics and the Weeping Angels.
iii) date specified in canon
24 April 2010: marriage of Bernice Summerfield and Jason Kane, as described in Paul Cornell's 1996 novel Happy Endings. (I don't think it's a huge spoiler to reveal that things didn't really work out between them.)