i) births and deaths

21 April 1927: birth of Gerald Flood, who played Kamelion in 1983 and 1984, and also King John in The King's Demons (Fifth Doctor, 1983)

21 April 1935: birth of Anthony Read, script editor of Doctor Who from Underworld (Fourth Doctor, 1978) to The Armageddon Factor (Fourth Doctor, 1979), co-writer of The Invasion of Time (Fourth Doctor, 1978) and writer of The Horns of Nimon (Fourth Doctor, 1979-80).

21 April 1973: birth of Mark Dexter, who played Charlotte's Dad in Silence in the Library/Forest of the Dead (Tenth Doctor, 2008) and Charles Babbage in Spyfall (Thirteenth Doctor, 2020).

21 April 1983: birth of Gugu Mbatha-Raw, who played Martha Jones' sister Tish in 2006.

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 1996: last ever appearance on TV of Jon Pertwee, playing the Third Doctor on Cilla Black's Surprise, Surprise!

21 April 2005: David Tennant films his half of the Ninth/Tenth Doctor regeneration scene.

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.