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 Who-linked fiction
12 December 1901: John Lafayette is transported to ancient Babylon, in Kate Orman's novel Walking to Babylon (1998) - yet another December date in one of her books. I can't remember if the date is also specified in the audio version, where he is played by Barnaby Edwards and the High Priestess by Elisabeth Sladen.