19 April 2011: death of the much missed Elisabeth Sladen, who played Sarah Jane Smith (companion to Third and Fourth Doctors, 1973-76; various appearances from then on, culminating in her own series from 2007 to 2011).
ii) broadcast anniversaries
19 April 1969: broadcast of first episode of The War Games. The Tardis lands in what appears to be a first world war battlefield, and the Doctor is sentenced to death by firing squad.
19 April 1975: broadcast of first episode of Revenge of the Cybermen. The Doctor, Harry and Sarah arrive at Space Beacon Nerva to find many dead crew members, a mysterious golden asteroid and Cybermats.
19 April 2002: webcast of "Death Comes to Time, Part 1", which is, confusingly, the eleventh episode of Death Comes to Time.
19 April 2008: broadcast of Planet of the Ood. The Doctor and Donna visit the Ood Sphere and liberate the Ood from their human masters.
19 April 2010: broadcast of The Black Hunger, fifteenth episode of the Australian K9 series. The Department is using an alien device which literally eats rubbish. Darius sees an opportunity to make some money and snatches the device. However the alien virus inside escapes and threatens to devour everything it touches.
19 April 2020: webcast of Farewell, Sarah Jane. Filmed in pandemic lockdown, shown on the ninth anniversary of Lis Sladen's death, this was elegiac and appropriate. (Given that Sacha Dhawan fimed the segments featuring his partner Anjli Mohindra, is this the first Doctor Who to be partly filmed by the Master?)
iii) date specified in-universe
19 April 2011: setting of most of Closing Time (Eleventh Doctor).