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.
The Deathsmiths of Goth are one of Alan Moore’s contributions to the non-televised canon; the Cybermen are researching them in his Doctor Who Weekly comic strip Black Legacy (1980), drawn in suitably heavy inks by David Lloyd.
ETA: The N-Forms appear in Russell T Davies’s New Adventure, Damaged Goods.