May 20 is the 140th day of the year in the Gregorian Calendar (141st in leap years). There are 225 days remaining in the year.

  1. 325 - The First Council of Nicaea – the first Ecumenical Council of the Christian Church – is held.
  2. 526 - An earthquake kills about 300,000 people in Syria and Antiochia.
  3. 685 - The Battle of Dunnichen or Nechtansmere is fought between a Pictish army under King Bridei III and the invading Northumbrians under King Ecgfrith, who are decisively defeated.
  4. 1217 - The Second Battle of Lincoln is fought near Lincoln, England, resulting in the defeat of Prince Louis of France by William Marshal, 2nd Earl of Pembroke.
  5. 1293 - King Sancho IV of Castile creates the Study of General Schools of Alcalá.
  6. 1497 - John Cabot sets sail from Bristol, England, on his ship Matthew looking for a route to the west (other documents give a 2 May date).
  7. 1498 - Portuguese explorer Vasco da Gama arrives at Kozhikode (previously known as Calicut), India.
  8. 1521 - Battle of Pampeluna: Ignatius Loyola seriously wounded in the battle.
  9. 1570 - Cartographer Abraham Ortelius issues the first modern atlas.
  10. 1631 - The city of Magdeburg in Germany is seized by forces of the Holy Roman Empire and most of its inhabitants massacred, in one of the bloodiest incidents of the Thirty Years' War.
  11. 1690 - England passes the Act of Grace, forgiving followers of Roman Catholic James II.
  12. 1813 - Napoleon Bonaparte leads his French troops into the Battle of Bautzen in Saxony, Germany, against the combined armies of Russia and Prussia. The battle ends the next day with a French victory.
  13. 1845 - HMS Erebus and HMS Terror with 134 men under John Franklin sail from the River Thames in England, beginning a disastrous expedition to find the Northwest Passage. All hands are lost.
  14. 1861 - American Civil War: The state of Kentucky proclaims its neutrality, which will last until September 3 when Confederate forces enter the state.
  15. 1861 - American Civil War: The state of North Carolina secedes from the Union, joining The Confederacy.
  16. 1862 - U.S. President Abraham Lincoln signs the Homestead Act into law.
  17. 1864 - American Civil War: Battle of Ware Bottom Church - In the Virginia Bermuda Hundred Campaign, 10,000 troops fight in this Confederate victory.
  18. 1873 - Levi Strauss and Jacob Davis receive a U.S. patent for blue jeans with copper rivets.
  19. 1882 - The Triple Alliance between Germany, Austria-Hungary and Italy is formed.
  20. 1883 - The eruption of Krakatoa begins, leading ultimately to the volcano's destruction three months later.
  21. 1891 - History of cinema: First public display of Thomas Alva Edison's prototype kinetoscope (shown at Edison's Laboratory for a convention of the National Federation of Women's Clubs).
  22. 1896 - The six ton chandelier of the Palais Garnier falls on the crowd resulting in the death of one and the injury of many others.
  23. 1902 - Cuba gains independence from the United States. Tomás Estrada Palma becomes the first President of Cuba.
  24. 1910 - Krazy Kat and Ignatz first appear in 'The Dingbat Family' comic strip
  25. 1916 - The Saturday Evening Post publishes its first cover with a Norman Rockwell painting ("Boy with Baby Carriage").
  26. 1918 - The small town of Codell, Kansas is struck by a tornado. Incredibly, the same town was also hit in 1916 and 1917 on this date
  27. 1920 - The Weimarer Nationalversammlung, the national assembly of Germany's Weimar Republic, is permanently dissolved.
  28. 1920 - Montreal Quebec station XWA broadcasts the first regularly scheduled radio programming in North America.
  29. 1927 - By the Treaty of Jedda, the United Kingdom recognizes the sovereignty of King Ibn Saud in the Kingdoms of Hejaz and Nejd, which later merged to become the Kingdom of Saudi Arabia.
  30. 1927 - At 07:52 Charles Lindbergh takes off from Roosevelt Field in Long Island, New York, on the world's first solo nonstop flight across the Atlantic Ocean, touching down at Le Bourget Field in Paris at 22:22 the next day.
  31. 1932 - Amelia Earhart takes off from Newfoundland to begin the world's first solo nonstop flight across the Atlantic Ocean by a female pilot, landing in Ireland the next day.
  32. 1940 - Holocaust: The first prisoners arrive at a new concentration camp at Auschwitz.
  33. 1941 - World War II: Battle of Crete – German paratroops invade Crete.
  34. 1949 - In the United States of America, the AFSA (the predecessor of the NSA) is established.
  35. 1954 - Chiang Kai-shek is selected for another term as President of the Republic of China by the National Assembly
  36. 1965 - PIA Flight 705, a Pakistan International Airlines Boeing 720 - 040 B crashes while descending to land at Cairo International Airport, killing 119 of the 125 passengers and crew.
  37. 1969 - The Battle of Hamburger Hill in Vietnam ends.
  38. 1980 - In a Referendum in Quebec, the population rejects by a 60% vote the proposal from its government to move towards independence from Canada.
  39. 1983 - First publications of the discovery of the virus that causes AIDS in the journal Science by Luc Montagnier[1] and Robert Gallo[2] individually.
  40. 1985 - Radio Martí, part of the Voice of America service, begins broadcasting to Cuba.
  41. 1989 - The Chinese authorities declare martial law in the face of pro-democracy demonstrations, setting the scene for the Tiananmen Square massacre.
  42. 1990 - The first post-Communist presidential and parliamentary elections are held in Romania.
  43. 1995 - In a second Referendum in Quebec, the population rejects by a slight majority the proposal from its government to move towards independence from Canada.
  44. 1996 - Gay rights: The Supreme Court of the United States rules in Romer v. Evans against a law that would have prevented any city, town or county in the state of Colorado from taking any legislative, executive, or judicial action to protect the rights of homosexuals.
  45. 2002 - East Timor becomes independent from Indonesian rule.
  46. 2005 - The world's tallest and fastest roller coaster, Kingda Ka, opens at Six Flags Great Adventure.
  47. 2006 - In Athens, Greece, Lordi wins the 51st Eurovision Song Contest representing Finland with the song "Hard Rock Hallelujah".