December 21 is the 355th day of the year (356th in leap years) in the Gregorian calendar. There are 10 days remaining until the end of the year. This is a frequent day for the winter solstice to occur in the northern hemisphere and summer solstice to occur in the southern hemisphere.
- 1140 – Conrad III of Germany besieged Weinsberg.
- 1598 – Battle of Curalaba: The revolting Mapuche, led by cacique Pelentaru, inflict a major defeat on Spanish troops in southern Chile.
- 1620 – Plymouth Colony: William Bradford and the Mayflower Pilgrims land on what is now known as Plymouth Rock in Plymouth, Massachusetts.
- 1832 – Egyptian–Ottoman War: Egyptian forces decisively defeat Ottoman troops at the Battle of Konya.
- 1844 – The Rochdale Pioneers commence business at their cooperative in Rochdale, England, starting the Cooperative movement.
- 1861 – Medal of Honor: Public Resolution 82, containing a provision for a Navy Medal of Valor, is signed into law by President Abraham Lincoln.
- 1872 – Challenger expedition: HMS Challenger, commanded by Captain George Nares, sails from Portsmouth.
- 1879 – World première of Henrik Ibsen’s A Doll’s House at the Royal Theatre in Copenhagen.
- 1883 – The first Permanent Force cavalry and infantry regiments of the Canadian Army are formed: The Royal Canadian Dragoons and The Royal Canadian Regiment.
- 1913 – Arthur Wynne’s “word-cross”, the first crossword puzzle, is published in the New York World.
- 1937 – Snow White and the Seven Dwarfs, the world’s first full-length animated feature, premieres at the Carthay Circle Theater.
- 1941 – World War II: A formal treaty of alliance between Thailand and Japan is signed in the presence of the Emerald Buddha in Wat Phra Kaew.
- 1962 – Rondane National Park is established as Norway’s first national park.
- 1967 – Louis Washkansky, the first man to undergo a heart transplant, dies in Cape Town, South Africa, after living for 18 days after the transplant.
- 1968 – Apollo program: Apollo 8, the first manned mission to the moon, is launched from the Kennedy Space Center in Florida. The crew performs the first ever manned Trans Lunar Injection and become the first humans to leave Earth’s gravity.
- 1969 – The Gay Activists Alliance is formed in New York City.
- 1969 – The United Nations adopts the Convention on the Elimination of All Forms of Racial Discrimination.
- 1973 – The Geneva Conference on the Arab-Israeli conflict opens.
- 1979 – Lancaster House Agreement: An independence agreement for Rhodesia is signed in London by Lord Carrington, Sir Ian Gilmour, Robert Mugabe, Joshua Nkomo, Bishop Abel Muzorewa and S.C. Mundawarara.
- 1988 – A bomb explodes on board Pan Am flight 103 over Lockerbie, Dumfries and Galloway, Scotland, killing 270.
- 1992 – A Dutch DC-10, flight Martinair MP 495, crashes at Faro Airport, killing 56 people.
- 1994 – Mexican volcano Popocatepetl, dormant for 47 years, erupts gases and ash.
- 1995 – The city of Bethlehem passes from Israeli to Palestinian control.
- 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.
- 2004 – Iraq War: A suicide bomber killed 22 at the forward operating base next to the main U.S. military airfield at Mosul, the single deadliest suicide attack on American soldiers.
- 2009 – Northwest Airlines Flight 253 was almost bombed by Al Qaida but the bomber was later apprehended. It is also known as the Christmas Day bombing attempt.