8 October 1966: broadcast of first episode of The Tenth Planet. The Tardis lands at the South Pole and the Doctor, Ben and Polly are apprehended by the staff of the base; and the Cybermen arrive.
also 8 October 1966: the very first regeneration sequence was filmed, for broadcast three weeks later, as William Hartnell transformed into Patrick Troughton; Hartnell's last filming in the lead role just a day under three years since his first.
Lovingly reconstructed in 2013:
8 October 1977: broadcast of second episode of The Invisible Enemy. First appearance of K-9!!! The Doctor and Leela travel to the Bi-Al research centre in the asteroid belt, get themselves cloned, miniaturised and injected into the Doctor's brain.
8 October 2007: broadcast of second episode of Eye of the Gorgon. Much running around with secret corridors and finally using a mirror to turn the Gorgon's power on itself, and transform Maria's father back to his usual self.