21 December

21 December was a date.

Holidays and observances
In the southern hemisphere on Earth, 21 December was also known as Midsummer's Eve. (PROSE: Iceberg) In the northern hemisphere, it was instead the winter solstice. While John Lennon was convinced that the date was also Aztec sun god Huitzilopochtli's birthday, the Seventh Doctor dismissed him as "getting [his] mythologies meddled". (PROSE: The Left-Handed Hummingbird)

Events
In 350 million BC, the Isop and Grem galaxies collided and formed the Silver Devastation. (PROSE: Time Traveller's Diary)

In 1956, the racial segregation of buses ended in Montgomery, Alabama, as a result of the Montgomery Bus Boycott. (TV: Rosa)

In 1981, the Hecate Cult intended to sacrifice Brendan Richards in Moreton Harwood. However, he was rescued by Sarah Jane Smith and K9 Mark III before this could happen. (TV: A Girl's Best Friend)

In 2010, it was the date of an unknown Torchwood Three operation. The date was in both Captain Jack Harkness' and Gwen Cooper's CIA files, detailing the figures of a "case" numbered "00938478766-04". (TV: The New World)

Other
According to a calendar on the Snowcap base, 21 December 1986 was a Sunday. (TV: The Tenth Planet)