December 6 is the 340th day of the year (341st in leap years) in the Gregorian calendar. There are 25 days remaining until the end of the year.

• 1534 – The city of Quito in Ecuador is founded by Spanish settlers led by Sebastián de Belalcázar.
• 1768 – The first edition of the Encyclopædia Britannica is published.
• 1790 – The U.S. Congress moves from New York City to Philadelphia, Pennsylvania.
• 1865 – The Thirteenth Amendment to the United States Constitution is ratified, banning slavery.
• 1877 – The first edition of the Washington Post is published.
• 1884 – The Washington Monument in Washington D.C. is completed.
• 1897 – London becomes the world's first city to host licenced taxicabs.
• 1917 – Halifax Explosion: In Canada, a munitions explosion kills more than 1,900 people and destroys part of the City of Halifax, Nova Scotia.
• 1947 – The Everglades National Park in Florida is dedicated.
• 1956 – A violent water polo match between Hungary and the USSR takes place during the 1956 Summer Olympics in Melbourne, against the backdrop of the 1956 Hungarian Revolution.
• 2005 – An Iranian Air Force C-130 military transport aircraft crashes into a ten-floor apartment building in a residential area of Tehran, killing all 84 on board and 44 more civilians.
• 2006 – NASA reveals photographs taken by Mars Global Surveyor suggesting the presence of liquid water on Mars.