12 December 1916: death of Michael Leeston-Smith, director of the story we now call The Myth Makers (First Doctor, 1965).
12 December 1938: birth of Leslie Schofield, who played Leroy in The War Games (Second Doctor, 1969) and Calib in The Face of Evil (Fourth Doctor, 1977).
12 December 1961: birth of Sarah Sutton, who played Nyssa, companion to the Fourth and Fifth Doctors, in 1981-83.
ii) broadcast anniversaries
12 December 1964: broadcast of "The End of Tomorrow", fourth episode of the story we now know as The Dalek Invasion of Earth. Barbara and Jenny break out of London in the truck; Susan likes the idea of a fresh start; Ian is menaced by the Slyther.
iii) date specified in-universe
12 December 1901: John Lafayette is transported to ancient Babylon, in Kate Orman's novel Walking to Babylon (1998). I can't remember if the date is also specified in the audio version (also 1998), where he is played by Barnaby Edwards and the High Priestess by Elisabeth Sladen.