June 20 is the 171st day of the year in the Gregorian calendar, 172nd when it is a leap year, then there are 194 left.
Its equivalent was usually 2 Messidor of the French Republican/Revolutionary calendar, officially referred to as Oat Day.
Like June 19 and 22, it is also a possible but infrequent date for the summer solstice in the terrestrial northern hemisphere and the winter solstice in the southern hemisphere and thus the respective beginning of summer and winter in each of the said two hemispheres, the most frequent date being June 21.
June 19 - June 20 - June 21
451: Attila is defeated in the Catalaunian fields near present-day Troyes in Champagne (great east of France).
1347: Charles de Blois, beaten, is taken prisoner during the battle of La Roche-Derrien.
1622: victory of Tilly, at the battle of Höchst, during the Thirty Years War.
1631: sack of Baltimore by Barbary pirates.
1783: Battle of Gondelour.
1789: Oath of the Jeu de Paume.
1791: attempted escape from France by its King Louis XVI, stopped with his family in Varennes.
1792: day of June 20, 1792, the Parisian people invaded the Tuileries Palace.
1862: Barbu Catargiu, Romanian Prime Minister, is assassinated.
1863: West Virginia becomes the 35th state of the United States.
1900: Assassination of Clemens von Ketteler precipitating the Boxer War.
1942: Kazimierz Piechowski manages to escape from the Auschwitz concentration camp.
1944: Decisive American victory, at the Battle of the Philippine Sea, during the War of the Pacific.
1954: Resolution No. 104, on a question submitted by Guatemala.
1956: Resolution No. 115 of the United Nations, for the admission of new members, including Morocco.
independence of the Federation of Mali.
Norodom Sihanouk takes an oath as "head of state" of the Kingdom of Cambodia, which allows him to exercise the powers of king without having the title.
1973: Ezeiza massacre, committed by Triple A snipers.
1991: Berlin again becomes the capital of Germany, in the process of reunification.
2001: Pervez Musharraf becomes President of Pakistan.
2010: victory of Juan Manuel Santos in the presidential election in Colombia.
opening of Rio+20.
Antónis Samarás becomes Prime Minister of Greece.
2015: attack in Graz.
2021: In Armenia, legislative elections are held to renew the members of the National Assembly, in an anticipated manner following the tensions caused by the Armenian defeat in the 2020 war in Nagorno-Karabakh. Prime Minister Nikol Pashinian's party wins the election.
Arts, culture and religion
1888: encyclical letter Libertas Præstantissimum from Pope Leo XIII on human freedom.
1975: Release of the film Jaws by Steven Spielberg.
2006: inauguration of the Museum of Ethnology and Primitive Arts on Quai Branly in Paris 15th at the instigation of the French President of the Republic Jacques Chirac.
Science and technology
1840: Patent granted to Samuel Morse for the invention of the electric telegraph.
1894: Bacteriologist Alexandre Yersin discovers the plague bacillus.
1908: The German Patent and Trademark Office grants a patent to Melitta Bentz, for her coffee filter.
1939: Erich Warsitz makes the first flight of a liquid-fueled rocket-powered aircraft [unclear] the Heinkel He 176.
1990: discovery of the asteroid (5261) Eureka.
Economy and Society
1946: opening of the "Le Lido" cabaret in Paris.
1966: foundation of SOS Médecins by the Parisian general practitioner Marcel Lascar.
1992: reintroduction in Estonia of the Estonian kroon, replacing the Russian ruble