i) births and deaths
20 July 1923: birth of James Bree, who played the Security Chief in The War Games (1969), Nefred the Decider in Full Circle (1980) and the Keeper of the Matrix in The Ultimate Foe (1986)
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 (2007).
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).