December 20 is the 354th day of the year (355th in leap years) in the Gregorian calendar; 11 days remain until the end of the year.
1803 – The Louisiana Purchase is completed at a ceremony in New Orleans.
1860 – South Carolina becomes the first state to secede from the United States with the South Carolina Declaration of Secession.
1915 – World War I: The last Australian troops are evacuated from Gallipoli.
1924 – Adolf Hitler is released from Landsberg Prison.
1940 – Captain America Comics #1, containing the first appearance of the superhero Captain America, is published.
1941 – World War II: First battle of the American Volunteer Group, better known as the 'Flying Tigers', in Kunming, China.
1942 – World War II: Japanese air forces bomb Calcutta, India.
1946 – It's a Wonderful Life premieres at the Globe Theatre in New York to mixed reviews 1951 – The EBR-1 in Arco, Idaho becomes the first nuclear power plant to generate electricity. The electricity powered four light bulbs.
1989 – The United States invasion of Panama deposes Manuel Noriega.
1991 – A Missouri court sentences the Palestinian militant Zein Isa and his wife Maria to death for the honor killing of their daughter Palestina.
2004 – A gang of thieves steal £26.5 million worth of currency from the Donegall Square West headquarters of Northern Bank in Belfast, Northern Ireland, United Kingdom, one of the largest bank robberies in British history.