| 143431 | December 21 | Event | 1951 – Libya becomes an independent country. |
| 143432 | December 21 | Event | 1962 – Rondane National Park is established as Norway's first national park. |
| 143433 | December 21 | Event | 1963 – The episode of intercommunal violence called "Bloody Christmas" begins in Cyprus, ultimately resulting in the displacement of 25,000-30,000 Turkish Cypriots and destruction of more than 100 villages. |
| 143434 | December 21 | Event | 1967 – Louis Washkansky, the first man to undergo a heart transplant, dies in Cape Town, South Africa, having lived for 18 days after the transplant. |
| 143435 | December 21 | Event | 1968 – Apollo program: Apollo 8 is launched from the Kennedy Space Center, placing its crew on a lunar trajectory for the first visit to another celestial body by humans. |
| 143436 | December 21 | Event | 1969 – The United Nations adopts the International Convention on the Elimination of All Forms of Racial Discrimination. |
| 143437 | December 21 | Event | 1973 – The Geneva Conference on the Arab–Israeli conflict opens. |
| 143438 | December 21 | Event | 1979 – Lancaster House Agreement: An independence agreement for Rhodesia is signed in London by Lord Peter Carrington, Sir Ian Gilmour, Robert Mugabe, Joshua Nkomo, Bishop Abel Muzorewa and S.C. Mundawarara. |
| 143439 | December 21 | Event | 1988 – A bomb explodes on board Pan Am Flight 103 over Lockerbie, Dumfries and Galloway, Scotland, killing 270. |
| 143440 | December 21 | Event | 1992 – A Dutch DC-10, flight Martinair MP 495, crashes at Faro Airport, killing 56. |
| 143441 | December 21 | Event | 1994 – Mexican volcano Popocatépetl, dormant for 47 years, erupts gases and ash. |
| 143442 | December 21 | Event | 1995 – The city of Bethlehem passes from Israeli to Palestinian control. |
| 143443 | December 21 | Event | 1999 – The Spanish Civil Guard intercepts a van loaded with 950 kg of explosives that ETA intended to use to blow up Torre Picasso in Madrid, Spain. |
| 143444 | December 21 | Event | 2004 – Iraq War: A suicide bomber killed 22 at the forward operating base next to the main U.S. military airfield at Mosul, Iraq, the single deadliest suicide attack on American soldiers. |
| 143445 | December 21 | Birth | 1118 – Thomas Becket, English archbishop and saint (d. 1170) |
| 143446 | December 21 | Birth | 1401 – Masaccio, Italian painter (d. 1428) |
| 143447 | December 21 | Birth | 1596 – Peter Mogila, Romanian-Ukrainian metropolitan (d. 1646) |
| 143448 | December 21 | Birth | 1603 – Roger Williams, English-American minister, theologian, and politician, 9th President of the Colony of Rhode Island (d. 1684) |
| 143449 | December 21 | Birth | 1672 – Benjamin Schmolck, German pastor and composer (d. 1737) |
| 143450 | December 21 | Birth | 1714 – John Bradstreet, Canadian-English general (d. 1774) |
| 143451 | December 21 | Birth | 1728 – Hermann Raupach, German harpsichord player and composer (d. 1778) |
| 143452 | December 21 | Birth | 1778 – Anders Sandøe Ørsted, Danish jurist and politician, 3rd Prime Minister of Denmark (d. 1860) |
| 143453 | December 21 | Birth | 1795 – John Russell, English hunter and dog breeder (d. 1883) |
| 143454 | December 21 | Birth | 1795 – Leopold von Ranke, German historian, author, and academic (d. 1886) |
| 143455 | December 21 | Birth | 1803 – Achille Vianelli, Italian painter (d. 1894) |
| 143456 | December 21 | Birth | 1804 – Benjamin Disraeli, English lawyer and politician, Prime Minister of the United Kingdom (d. 1881) |
| 143457 | December 21 | Birth | 1805 – Thomas Graham, Scottish chemist and academic (d. 1869) |
| 143458 | December 21 | Birth | 1811 – Archibald Tait, Scottish-English archbishop (d. 1882) |
| 143459 | December 21 | Birth | 1815 – Thomas Couture, French painter and educator (d. 1879) |
| 143460 | December 21 | Birth | 1818 – Amalia of Oldenburg (d. 1875) |