1820 was a leap year starting on Saturday (see link for calendar).

Events

• January 1 - Constitutionalist military insurrection at Cádiz leads to summoning of Spanish parliament (March 7) and restoration of 1812 Constitution (March 8) by king Ferdinand VII.
• January 28 - Russian expedition lead by Fabian Gottlieb von Bellingshausen and Mikhail Petrovich Lazarev approaches the Antarctic coast.
• January 29 - George the Prince Regent becomes king George IV of the United Kingdom, ending the period known as the English Regency.
• January 30 - Edward Bransfield lands on the Antarctic mainland.
• February 6 - 86 free African American colonists sail from New York City to Freetown, Sierra Leone.
• February 23 - The Cato Street conspiracy is exposed. The principals are executed on May 1
• March 3 & 6 - The Missouri Compromise becomes law in the United States.
• March 15 - Maine is admitted as the 23rd U.S. state.
• May 1 - Last hanging drawing and quartering in Britain – Cato Street conspirators for treason (only hanged and beheaded)
• Spring - Joseph Smith, Jr. at age 14 claims to be visited in a vision by God and Jesus (Tradition holds that this occurred on April 6)
• July - Constitutionalist revolution in the Kingdom of the Two Sicilies
• August 24 - Constitutionalist insurrection at Oporto, Portugal; revolution in Lisbon, September 15
• September 28 -In Salem, Massachusetts, Colonel Robert Gibbon Johnson eats a tomato in public to prove it is not poisonous.
• October 9 - Guayaquil declare independence from Ecuador.
• October 25-November 20 - Congress of Troppau (Opava) between rulers of Russia, Austria and Prussia
• November - James Monroe is re-elected, virtually unopposed.
• November 17 - Captain Nathaniel Palmer becomes the first American to see Antarctica (the Palmer Peninsula was later named after him).

unknown date

• 6th Edition of Encyclopædia Britannica begins appearing.
• Republic of Buenos Aires (Argentina) establishes penal colony in Falkland Islands.
• Venus de Milo found on island of Melos.
• Hans Christian Ørsted discovers relationship between electricity and magnetism.

Births

• January 17 - Anne Brontë, English author (d. 1849)
• February 8 - William Tecumseh Sherman, soldier (d. 1891)
• February 15 - Susan B. Anthony, American suffragist (d. 1906)
• February 17 - Henri Vieuxtemps, Belgian composer (d. 1881)
• February 28 - John Tenniel, English illustrator (d. 1914)
• March 14 - Victor Emmanuel II of Italy (d. 1878)
• May 12 - Florence Nightingale, English nurse (d. 1910)
• May 27 - Mathilde Bonaparte, hostess and socialite (d. 1904)
• July 23 - Julia Gardiner Tyler, First Lady of the United States (d. 1889)
• September 27 - Wilhelm Siegmund Teuffel, German classical scholar (d. 1878)
• September 29 - Henry, Duke of Bordeaux, posthumous son of the Duke of Berry, future Comte de Chambord and claimant to the French throne (d. 1883)
• 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)
• Harriet Tubman, American anti-slavery resistance movement leader (d. 1913)

Deaths

• January 29 - King George III of the United Kingdom (b. 1738)
• February 14 - Charles Ferdinand, Duke of Berry, stabbed on February 13 (b. 1778)
• March 22 - Stephen Decatur, American sailor (b. 1779)
• June 19 - Sir Joseph Banks, British naturalist and botanist (b. 1743)
• September 3 - Benjamin Latrobe, English architect (b. 1764)