) was a leap year starting on Saturday
(link will display the full calendar) of the Gregorian calendar
(or a leap year starting on Thursday
of the 12-day slower Julian calendar
Events of 1820
- January 1 - A constitutionalist military insurrection at Cádiz leads to the summoning of the Spanish Parliament (March 7) (see Mid-nineteenth century Spain).
- January 28 - A Russian expedition led by Fabian Gottlieb von Bellingshausen and Mikhail Petrovich Lazarev approaches the Antarctic coast (see History of Antarctica).
- January 29 - George IV of the United Kingdom ascends the throne, ending the period known as the British Regency. There will be a gap of 21 years before the title Prince of Wales is next used.
- January 30 - Edward Bransfield lands on the Antarctic mainland (see History of Antarctica).
- February 6
- February 14 - Minh Mang starts to rule in Vietnam.
- February 20 - A revolt begins in Santa María Chiquimula, Totonicapán department of Guatemala.
- February 23 - The Cato Street conspiracy is exposed; the principals are executed on May 1.
- March 3 & 6 - Slavery in the United States: The Missouri Compromise becomes law.
- March 9 - King Ferdinand VII of Spain accepts the new constitution, beginning the Liberal Triennium ("Trienio Liberal").
- March 15 - Maine is admitted as the 23rd U.S. state.
- May 1 - The last hanging, drawing and quartering in Britain is meted out to the Cato Street conspirators for treason (only hanged and beheaded).
- May 11 - The HMS Beagle (the ship that later takes young Charles Darwin on his scientific voyage) is launched.
- July - A Constitutionalist revolution occurs in the Kingdom of the Two Sicilies.
- July 20 - Saint Cronan's Boys National School opens in Bray, Co. Wicklow, Ireland under the title Bray Male School. It is the oldest school in Bray and its notable past pupils include the former President of Ireland, Cearbhall Ó Dálaigh.
- August 24 - A Constitutionalist insurrection breaks out at Oporto, Portugal.
- September 15 - A revolution breaks out in Lisbon (see Portugal's crises of the Nineteenth Century).
- October 9 - Guayaquil declares independence from Spain (see also History of Ecuador).
- October 25-November 20 - The Congress of Troppau (Opava) is convened between the rulers of Russia, Austria and Prussia.
- December 3 - U.S. presidential election, 1820: James Monroe is re-elected, virtually unopposed.
January - June
- January 10 - Louisa Lane Drew, actress and prominent theater manager, grandmother of the Barrymores (d. 1897)
- January 17 - Anne Brontë, English author (d. 1849)
- February 1 - George Hendric Houghton, American Protestant Episcopal clergyman (d. 1897)
- February 8 - William Tecumseh Sherman, American Civil War general (d. 1891)
- February 13 - James Geiss, English businessman (d. 1878)
- February 15 - Susan B. Anthony, American suffragist (d. 1906)
- February 17 - Henri Vieuxtemps, Belgian violinist and composer (d. 1881)
- February 28 - John Tenniel, English illustrator (d. 1914)
- March 3 - Henry D. Cogswell, American philanthropist and temperance movement pioneer (d. 1900)
- March 4 - Francesco Bentivegna, Italian revolutionary (d. 1856)
- March 20 - Alexandru Ioan Cuza, Romania's first reigning Domnitor (d. 1873)
- March 14 - Victor Emmanuel II of Italy (d. 1878)
- April 27 - Herbert Spencer, English philosopher (d. 1903)
- May 12 - Florence Nightingale, English nurse (d. 1910)
- May 23 - Lorenzo Sawyer, 9th Chief Justice of the Supreme Court of California (d. 1891)
- May 27 - Mathilde Bonaparte, Italian princess (d. 1904)
July - December
January - June
July - December