March 31 is the 90th day of the year (91st in leap years) in the Gregorian calendar. There are 275 days remaining until the end of the year.
* 307 – After divorcing his wife Minervina, Constantine marries Fausta, the daughter of the retired Roman Emperor Maximian.
* 1146 – Bernard of Clairvaux preaches his famous sermon in a field at Vézelay, urging the necessity of a Second Crusade. Louis VII is present, and joins the Crusade.
* 1492 – Queen Isabella of Castille issues the Alhambra decree, ordering her 150,000 Jewish subjects to convert to Christianity or face expulsion.
* 1717 – A sermon on "The Nature of the Kingdom of Christ" by Benjamin Hoadly, the Bishop of Bangor, provokes the Bangorian Controversy.
* 1774 – American Revolutionary War: The Kingdom of Great Britain orders the port of Boston, Massachusetts closed pursuant to the Boston Port Act.
* 1822 – The massacre of the population of the Greek island of Chios by soldiers of the Ottoman Empire following a rebellion attempt, depicted by the French artist Eugène Delacroix.
* 1854 – Commodore Matthew Perry signs the Treaty of Kanagawa with the Japanese government, opening the ports of Shimoda and Hakodate to American trade.
* 1866 – The Spanish Navy bombs the harbor of Valparaíso, Chile.
* 1877 – The family with samurai antecedents that responded to the Saigo army in Ōita Nakatsu rebels.
* 1885 – The United Kingdom establishes a protectorate over Bechuanaland.
* 1889 – The Eiffel Tower is inaugurated.
* 1903 – Richard Pearse allegedly makes a powered flight in an early aircraft.
* 1906 – The Intercollegiate Athletic Association of the United States (later National Collegiate Athletic Association) is established to set rules for amateur sports in the United States.
* 1909 – Serbia accepts Austrian control over Bosnia-Herzegovina.
* 1909 – Construction begins on the RMS Titanic.
* 1910 – Six North Staffordshire Pottery towns federate to form modern Stoke-on-Trent
* 1912 – Construction is completed on the RMS Titanic.
* 1917 – The United States takes possession of the Danish West Indies after paying $25 million to Denmark, and renames the territory the United States Virgin Islands.
* 1918 – Daylight saving time goes into effect in the United States for the first time.
* 1921 – The Royal Australian Air Force is formed.
* 1930 – The Motion Pictures Production Code is instituted, imposing strict guidelines on the treatment of sex, crime, religion and violence in film, in the U.S., for the next thirty eight years.
* 1931 – An earthquake destroys Managua, Nicaragua, killing 2,000.
* 1933 – The Civilian Conservation Corps is established with the mission of relieving rampant unemployment.
* 1942 – World War II: Japanese forces invade Christmas Island, then a British possession.
* 1942 – Holocaust in Ivano-Frankivsk (then called Stanislawow), western Ukraine. German Gestapo organize the first deportation of 5,000 Jews from Stanislawow ghetto to Belzec death camp. It is one of the largest transports to Belzec in the first phase of the camp.
* 1946 – The first election is held in Greece after World War II.
* 1949 – The Dominion of Newfoundland joins the Canadian Confederation and becomes the 10th Province of Canada.
* 1951 – Remington Rand delivers the first UNIVAC I computer to the United States Census Bureau.
* 1957 – Elections to the Territorial Assembly of the French colony Upper Volta are held. After the elections PDU and MDV form a government.
* 1959 – The 14th Dalai Lama, Tenzin Gyatso, crosses the border into India and is granted political asylum.
* 1964 – The Dictatorship in Brazil, under the aegis of general Castello Branco, begins.
* 1965 – Iberia Airlines Convair 440 crashes into the sea on approach to Tangier, killing 47 of 51 occupants.
* 1966 – The Soviet Union launches Luna 10 which later becomes the first space probe to enter orbit around the Moon.
* 1968 – President Lyndon B. Johnson announces he will not run for re-election.
* 1970 – Explorer 1 re-enters the Earth's atmosphere (after 12 years in orbit).
* 1970 – Nine terrorists from the Japanese Red Army hijack Japan Airlines Flight 351 at Tokyo International Airport, wielding samurai swords and carrying a bomb.
* 1979 – The last British soldier leaves the Maltese Islands. Malta declares its Freedom Day (Jum il-Helsien).
* 1980 – The Chicago, Rock Island and Pacific railroad operates its final train after being ordered to liquidate its assets because of bankruptcy and debts owed to creditors.
* 1985 – The first WrestleMania, the biggest wrestling event from the WWE, takes place in Madison Square Garden in New York
* 1986 – A Mexicana Boeing 727 en route to Puerto Vallarta erupts in flames and crashes in the mountains northwest of Mexico City, killing 166.
* 1986 – Six metropolitan county councils are abolished in England.
* 1990 – 200,000 protestors take to the streets of London to protest against the newly introduced Poll Tax.
* 1991 – The Islamic Constitutional Movement, or Hadas, is established in Kuwait.
* 1991 – Georgian independence referendum, 1991: nearly 99 percent of the voters support the country's independence from the Soviet Union.
* 1992 – The USS Missouri (BB-63), the last active United States Navy Battleship, is decommissioned in Long Beach, California.
* 1994 – Human evolution: The journal Nature reports the finding in Ethiopia of the first complete Australopithecus afarensis skull.
* 1995 – In Corpus Christi, Texas, Latin superstar Selena Quintanilla Perez is shot and killed by Yolanda Saldivar, the president of her own fan club.
* 1998 – Netscape releases the code base of its browser under an open-source license agreement; the project is given the code name Mozilla and would eventually be spun off into the non-profit Mozilla Foundation.
* 2004 – In Fallujah, Iraq, 4 American private military contractors working for Blackwater USA, are killed and their bodies mutilated after being ambushed.
* 2007 – In Sydney, Australia, 2.2 million people take part in the first Earth Hour.
* 2008 – Aloha Airlines, a bankrupt airline, permanently ends passenger service