January 20 is the 20th day of the year in theGregorian calendar. There are 345 days remaining until the end of the year (346 in leap years).
- 1265 – In Westminster, the first English parliamentconducts its first meeting held by Simon de Montfort in the Palace of Westminster, now also known colloquially as the "Houses of Parliament".
- 1576 – The Mexican city of León is founded by order of the viceroy Don Martín Enríquez de Almanza.
- 1783 – The Kingdom of Great Britain signs a peace treaty with France and Spain, officially ending hostilities in the American Revolutionary War (also known as the American War of Independence).
- 1801 – John Marshall is appointed the Chief Justice of the United States.
- 1841 – Hong Kong Island is occupied by the British
- 1885 – L.A. Thompson patents the roller coaster.
- 1887 – The United States Senate allows the Navy to lease Pearl Harbor as a naval base.
- 1972 – Pakistan launched its Nuclear detterent program few weeks after its defeat in Bangladesh Liberation War and Indo-Pakistani War of 1971.
- 1981 – Twenty minutes after Ronald Reagan is inaugurated, at age 69 the oldest man ever to be inaugurated as U.S. President, Iran releases 52 American hostages.
- 1999 – The China News Service announces new government restrictions onInternet use, aimed especially at Internet cafés.
- 2006 – Witnesses report seeing a bottlenose whale swimming in the River Thames, the first time the species had been seen in the Thames since records began in 1913.
- 2007 – A three-man team, using only skis and kites, completes a 1,093-mile (1,759 km) trek to reach the southern pole of inaccessibility for the first time since 1958 and for the first time ever without mechanical assistance.