30 April 1983: death of Gabor Baraker, who played Wang-Lo, the rather camp tavern-keeper, in the story we now call Marco Polo and Luigi Ferrigo, a dodgy Genoese merchant, in the story we now call The Crusade
30 April 2004: death of Richard Steele, who played Commandant Gorton in The War Games (Second Doctor, 1969), Sergeant Hart in Doctor Who and the Silurians (Third Doctor, 1970) and a guard in Mark of the Rani (Sixth Doctor, 1985).
30 April 2020: I missed the sad news a year ago of the death of Wally K. Daly, who wrote The Ultimate Evil for the Sixth Doctor but never saw it produced; he did write the novelisation and eventually authorised Big Finish to produce an audio version. I've heard some of his other radio plays, which are great.
ii) broadcast anniversaries
30 April 1966: broadcast of "A Holiday for the Doctor", first episode of the story we now call The Gunfighters. The Doctor, Steven and Dodo arrive in Tombstone, Arizona in 1881 in search of a dentist.
30 April 2005: broadcast of Dalek. The Doctor and Rose encounter a captive Dalek in 2012 America.
30 April 2011: broadcast of Day of the Moon. Against the backdrop of the first moon landing, the Doctor and his companions must solve the mystery of the aliens and the little girl.
30 April 2020: webcast of Sven's Scarf, a prequel to Dalek.
iii) date almost specified in canon
30 April, some time in the 1970s: most of the events of The Dæmons (1971).
Two more months to go of this project. It's been a fun distraction in strange times.