Year 1820 (MDCCCXX) 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 - Constitutionalist military insurrection at Cádiz leads to summoning of Spanish parliament (March 7) (See Mid-nineteenth century Spain.)
• January 28 - Russian expedition lead 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 twenty one 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
  • Lord Thomas Cochrane occupies Valdivia in name of the republic of Chile.
  • 86 free African American colonists sail from New York City to Freetown, Sierra Leone.
• February 8 - William Tecumseh Sherman is born.
• 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.
• April
  • Hans Christian Ørsted discovers the relationship between electricity and magnetism.
  • Radical War is fought in Scotland.
  • Church of Jesus Christ of Latter-day Saints is founded.
• May 1 - Last hanging drawing and quartering in Britain – Cato Street conspirators for treason (only hanged and beheaded.)
• May 11 - Launch of HMS Beagle the ship that would later take young Charles Darwin on his scientific voyage.
• July - Constitutionalist revolution 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 former President of Ireland, Cearbhall Ó Dálaigh
• August 24 - Constitutionalist insurrection at Oporto, Portugal; revolution in Lisbon, September 15 (see Portugal's crises of the Nineteenth Century.).
• October 9 - Guayaquil declare independence from Spain. (See also History of Ecuador).
• October 25-November 20 - Congress of Troppau (Opava) between rulers of Russia, Austria and Prussia.
• December 3 - U.S. presidential election, 1820 - James Monroe is re-elected, virtually unopposed.

Undated

• The 6th Edition of Encyclopædia Britannica appears.
• The Argentine Confederation (Argentina) establishes a penal colony in Falkland Islands.
• Venus de Milo is found on the island of Melos (Milos).
• Robert Owen devises the labour voucher.
• Mount Rainier erupts over what is today Seattle.
• Joseph Smith, Jr. experiences his First Vision in spring in Palmyra, New York.
• 18,957 black slaves leave Luanda.

Births

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,First Romania's reigning (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

• July 9 - John Wright Oakes, Landscape painter (d. 1887)
• July 23 - Julia Gardiner Tyler, First Lady of the United States (d. 1889)
• September 17 - Émile Augier, French dramatist (d. 1889)
• September 27 - Wilhelm Siegmund Teuffel, German classical scholar (d. 1878)
• September 29 - Comte de Chambord, claimant to the French throne (d. 1883)
• October 5 - David Wilber, American politician (d. 1890)
• October 6 - Jenny Lind, Swedish soprano (d. 1887)
• November 23 - Isaac Todhunter, English mathematician (d. 1884)
• November 28 - Friedrich Engels, German social philosopher (d. 1895)

Deaths

January - June

• January 17 - Daniel Albert Wyttenbach, academic (b. 1746)
• January 29 - King George III of the United Kingdom (b. 1738)
• February 11 - Karl von Fischer, architect (b. 1782)
• February 14 - Charles Ferdinand, Duke of Berry (stabbed) (b. 1778)
• March 22 - Stephen Decatur, American sailor (b. 1779)
• May 30 - William Bradley, Britain's tallest ever man (b. 1787)
• June 9 - Wilhelmina of Prussia, Princess of Orange (b. 1751)
• June 19 - Sir Joseph Banks, British naturalist and botanist (b. 1743)

July - December

• August 12 - Manuel Lisa, fur trader (b. 1772)
• September 3 - Benjamin Latrobe, English architect (b. 1764)
• September 26 - Daniel Boone, American pioneer (b. 1734)
• September 29 - Barthelemy Lafon, architect and smuggler (b. 1769)
• October 15 - Karl Philipp Fürst zu Schwarzenberg, Austrian field marshal (b. 1771)
• December 25 - Joseph Fouché, French statesman (b. 1763)
• date unknown - William Drennan, Northern Irish poet (b. 1754)

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