21 April 1927: birth of Gerald Flood, who played Kamelion in 1983 and 1984, and also King John in The King’s Demons (1983).
21 April 1935: birth of Anthony Read, script editor of Doctor Who from Underworld (1978) to The Armageddon Factor (1979), co-writer of The Invasion of Time (1978) and writer of The Horns of Nimon (1979-80).
21 April 2002: death of Terry Walsh, stuntman and actor in many Old Who stories.
ii) broadcast anniversaries
21 April 1973: broadcast of third episode of Planet of the Daleks. The Doctor and the Thals escape from the Dalek base.
21 April 2007: broadcast of Daleks in Manhattan. The Doctor and Martha land in New York in 1930 where the Daleks are attempting to merge with humans.
Dark Eden, Eva, and the Marilynne Robinsons are my tips.