i) births and deaths
20 July 1923: birth of James Bree, who played the Security Chief in The War Games (Second Doctor, 1969), Nefred the Decider in Full Circle (Fourth Doctor, 1980) and the Keeper of the Matrix in The Ultimate Foe (Sixth Doctor, 1986).
20 July 1931: birth of Roy Skelton, whose first voice work was for the Monoids in The Ark (First Doctor, 1966) and did Dalek voices all the way from Evil of the Daleks (Second Doctor, 1967) to The Curse of Fatal Death (alternate Doctors, 1999). He had three on-screen roles as well, Wilfred Norton in Colony in Space (Third Doctor, 1972), James the chemicals man in The Green Death (Third Doctor, 1973), a role hastily invented when another actor fell ill halfway through filming, and King Rokon of the Kastrians in The Hand of Fear (Fourth Doctor, 1976).
20 July 1951: birth of Jeff Rawle, who played leadership heir Plantagenet in Frontios (Fifth Doctor, 1984), gallery director Lionel Harding in Mona Lisa's Revenge (Sarah Jane Adventures, 2009) and early Doctor Who director Mervyn Pinfield in An Adventure in Space and Time (2013).
20 July 1998: birth of Sinead Michael, who became part of the regular cast of the Sarah Jane Adventures in its truncated final season (2010). At twelve, she is the youngest actor to have been in the regular cast of any show in the Whoniverse.
ii) broadcast anniversaries - none
iii) dates specified in canon
20 July 1966: the day that the First Doctor leaves London with Ben and Polly in The War Machines (1966) and the Second Doctor brings them back in The Faceless Ones (1967), only to find that he and Jamie need to find the stolen Tardis in Evil of the Daleks (also 1967).
20 July 1969: Neil Armstrong walks on the moon (21 July European time), an event Martha Jones says she saw four times in Blink (Tenth Doctor, 2007) and we all see it in Day of the Moon (Eleventh Doctor, 2011).
20 July 2006: forty years on from her depature and return, Polly Wright emails Sir Alastair Lethbridge-Stewart as a result of reading an article about Jo Jones (née Grant) in the first installment of The Three Companions audioplay (released in 12 monthly episodes 2009-10).