i) births and deaths
Three sad notes today.
21 July 1995: death of Michael Wisher, who appeared as TV anchor Wakefield in The Ambassadors of Death (Third Doctor, 1970); factory owner Rex Farrel in Terror of the Autons (Third Doctor, 1971); Kalik in Carnival of Monsters (Third Doctor, 1973); most iconically as Davros in Genesis of the Daleks (Fourth Doctor, 1975); Magrik of the Vogans in Revenge of the Cybermen (also Fourth Doctor, 1975); Morelli in Planet of Evil (yet again, Fourth Doctor, 1975 - his third role in four stories); and Robar the ship's engineer in Shakedown (unofficial, 1994). A man of many faces, as you can see.
21 July 1998: Not quite as significant as the other two, but worth noting the death of Kenneth Watson, who played Craddock in Daleks' Invasion Earth 2150 A.D. (Cushing movie, 1966) and engineer Duggan in The Wheel in Space (Second Doctor, 1968).
21 July 2017: Death of Deborah Watling, who played the Second Doctor's companion Victoria Waterfield in 1967-8 (and also returned for the unofficial spinoff Downtime in 1998).
ii) publication date
21 July 1994: publication of the first of the Virgin Missing Adventures, Paul Cornell's tale of the Fifth Doctor and vampires, Goth Opera. I won't normally track publication dates here but this is a significant event in the development of Who books, the first spinoff book featuring a non-current Doctor.
iii) date specified in-universe
21 July 2006: UNIT locks its files on Judoon translators (and by implication Judoon torches also). See The Secret Life of Monsters (Twelfth Doctor book, 2014 - a particularly good one).