28 April 1977: death of Anthony Coburn, who wrote An Unearthly Child (1963) and the never-produced story The Masters of Luxor.
i) broadcast anniversaries
28 April 1973: broadcast of fourth episode of Planet of the Daleks. The Doctor and Jo are reunited, and the Thals disagree about how to tackle the Daleks.
28 April 2007: broadcast of Evolution of the Daleks. The Daleks' experiments are destroyed by the Doctor and Dalek Caan escapes.
ii) date specified in canon
28 April 2008: Martha Jones and Sarah Jane Smith are killed on the Moon, in the negated timeline in Turn Left (2008)