i) births and deaths

20 December 1943: birth of Jacqueline Pearce, who played Chessene in The Two Doctors (1985), Prime Miniister Sherilyn Harper in Big Finish audio The Fearmonger (2000), Admiral Mettna in the webcast Death Comes to Time (2001-02) and of course Servalan in Blake's 7.

20 December 1978: birth of Eddie Robson, author of many Big Finish audios and various short stories.

ii) broadcast anniversaries

This is the first day since 28 August (arguably 21 August) with no broadcast anniversaries, and the last such day in the calendar year.

iii) date specified in canon

20 December 1992: St Christopher's Church in Cheldon Bonniface is transported to the Moon, as recounted in Paul Cornell's 1991 novel Timewyrm: Revelation.