i) births and deaths
26 July 1934: birth of Neil McCarthy, who played prisoner Barnham in The Mind of Evil (Third Doctor, 1971) and methane refinery controller Thawn in The Power of Kroll (Fourth Doctor, 1978).
26 July 1939: birth of Bob Baker, who wrote eight classic Old Who stories with Dave Martin and one (Nightmare of Eden) without him, as well as two episodes of the Australian K9 series. (He's still alive. Happy 81st birthday, Bob!)
26 July 1978: It seems I made a mistake a couple of weeks ago, and today is the real birthday of Eve Myles, who plays Gwen Cooper in Torchwood (and also appeared in The Unquiet Dead with the Ninth Doctor in 2005). I'm sorry, that means I will have to post this picture again..
26 July 2007: death of John Normington, who most notably played Morgus in The Caves of Androzani (Fifth Doctor, 1984) but also Trevor Sigma, the Galactic Censor in The Happiness Patrol (Seventh Doctor, 1988) and Tom Flanagan, evacuated from London to Cardiff as a child and interviewed by Gwen Cooper in Ghost Machine (Torchwood, 2008)
26 July 2012: death of Mary Tamm, who played the first Romana to the Fourth Doctor in 1978-79.
ii) broadcast / performance anniversary
26 July 1966: release of Daleks' Invasion Earth 2150 A.D., the second Peter Cushing film, based on the First Doctor story we now call The Dalek Invasion of Earth. Co-starring Bernard Cribbins who went on to do the TV programme four decades later.
iii) date of real event, implicitly specified in canon
26 July 1794: setting of the later part of "The Tyrant of France", all of "A Bargain of Necessity" and the first part of "Prisoners of Conciergerie", the fourth, fifth and sixth episodes of the story we now call The Reign of Terror (First Doctor, 1964). I'm sorry, I just love Hartnell in that costume.