April 1 is the 91st day of the year (92nd in leap years) in the Gregorian calendar. There are 274 days remaining until the end of the year. April 1 is most notable in the Western world for being April Fools' Day.
* 527 – Byzantine Emperor Justin I names his nephew Justinian I as co-ruler and successor to the throne.
* 1293 – Robert Winchelsey leaves England for Rome, to be consecrated as Archbishop of Canterbury.
* 1318 – Berwick-upon-Tweed is captured by the Scottish from the English.
* 1340 – Niels Ebbesen kills Gerhard III of Holstein in his bedroom, ending the 1332-1340 interregnum in Denmark.
* 1572 – In the Eighty Years' War, the Watergeuzen capture Brielle from the Spaniards, gaining the first foothold on land for what would become the Dutch Republic.
* 1789 – In New York City, the United States House of Representatives holds its first quorum and elects Frederick Muhlenberg of Pennsylvania as its first House Speaker.
* 1826 – Samuel Morey patents the internal combustion engine.
* 1854 – Hard Times begins serialisation in Charles Dickens' magazine, Household Words.
* 1857 – Herman Melville publishes The Confidence-Man.
* 1865 – American Civil War: Battle of Five Forks – In Siege of Petersburg, Confederate General Robert E. Lee begins his final offensive.
* 1867 – Singapore becomes a British crown colony.
* 1873 – The British steamer RMS Atlantic sinks off Nova Scotia, killing 547.
* 1887 – Mumbai Fire Brigade is established.
* 1891 – The Wrigley Company is founded in Chicago, Illinois.
* 1893 – The rank of Chief Petty Officer in the United States Navy is established.
* 1908 – The Territorial Force (renamed Territorial Army in 1920) is formed as a volunteer reserve component of the British Army.
* 1912 – The Greek athlete Konstantinos Tsiklitiras breaks the world record in the standing long jump with a distance of 3.47 meters.
* 1918 – The Royal Air Force is created by the merger of the Royal Flying Corps and the Royal Naval Air Service.
* 1924 – Adolf Hitler is sentenced to five years in jail for his participation in the "Beer Hall Putsch". However, he spends only nine months in jail, during which he writes Mein Kampf.
* 1924 – The Royal Canadian Air Force is formed.
* 1933 – The recently elected Nazis under Julius Streicher organize a one-day boycott of all Jewish-owned businesses in Germany, ushering in a series of anti-Semitic acts.
* 1936 – Orissa formerly known as Kalinga or Utkal becomes a state in India.
* 1937 – Aden becomes a British crown colony.
* 1937 – The power granted to the National Socialist government of Germany by the Enabling Act of 1933 officially expires, and is subsequently renewed by the Reichstag.
* 1939 – Generalísimo Francisco Franco of the Spanish State announces the end of the Spanish Civil War, when the last of the Republican forces surrender.
* 1941 – The Blockade Runner Badge for the German navy is instituted.
* 1944 – Navigation errors lead to an accidental American bombing of the Swiss city of Schaffhausen.
* 1945 – World War II: Operation Iceberg – United States troops land on Okinawa in the last campaign of the war.
* 1946 – Aleutian Island earthquake: A 7.8 magnitude earthquake near the Aleutian Islands creates a tsunami that strikes the Hawaiian Islands killing 159, mostly in Hilo.
* 1946 – Formation of the Malayan Union.
* 1948 – Cold War: Berlin Airlift – Military forces, under direction of the Soviet-controlled government in East Germany, set-up a land blockade of West Berlin.
* 1948 – Faroe Islands receive autonomy from Denmark.
* 1949 – Chinese Civil War: The Communist Party of China holds unsuccessful peace talks with the Kuomintang in Beijing, after three years of fighting.
* 1949 – The Canadian government repeals Japanese Canadian internment after seven years.
* 1949 – The twenty-six counties of the Irish Free State become the Republic of Ireland.
* 1954 – President Dwight D. Eisenhower authorizes the creation of the United States Air Force Academy in Colorado.
* 1955 – The EOKA rebellion against The British Empire begins in Cyprus, with the goal of obtaining the desired unification ("enosis") with Greece.
* 1957 – The BBC broadcasts the spaghetti tree hoax on its current affairs programme Panorama.
* 1967 – The United States Department of Transportation begins operation.
* 1969 – The Hawker Siddeley Harrier enters service with the Royal Air Force.
* 1970 – President Richard Nixon signs the Public Health Cigarette Smoking Act into law, requiring the Surgeon General's warnings on tobacco products and banning cigarette advertisements on television and radio in the United States, starting on January 1, 1971.
* 1973 – Project Tiger, a tiger conservation project, is launched in the Corbett National Park, India.
* 1974 – In the United Kingdom, the Metropolitan and non-metropolitan counties come into being.
* 1976 – Apple Inc. is formed by Steve Jobs and Steve Wozniak.
* 1976 – Conrail takes over operations from six bankrupt railroads in the Northeastern U.S..
* 1976 – The Jovian-Plutonian gravitational effect is first reported by British astronomer Patrick Moore.
* 1978 – The Philippine College of Commerce, through a presidential decree, becomes the Polytechnic University of the Philippines.
* 1979 – Iran becomes an Islamic Republic by a 98% vote, officially overthrowing the Shah.
* 1981 – Daylight saving time is introduced in the USSR.
* 1989 – Margaret Thatcher's new local government tax, the Community Charge (commonly known as the "poll tax"), is introduced in Scotland.
* 1992 – Start of the Bosnian war.
* 1996 – The Halifax Regional Municipality in Nova Scotia is created.
* 1997 – Comet Hale-Bopp is seen passing over perihelion.
* 1999 – Nunavut is established as a Canadian territory carved out of the eastern part of the Northwest Territories.
* 2001 – An EP-3E United States Navy surveillance aircraft collides with a Chinese People's Liberation Army Shenyang J-8 fighter jet. The Navy crew makes an emergency landing in Hainan, People's Republic of China and is detained.
* 2001 – Former President of Federal Republic of Yugoslavia Slobodan Milošević surrenders to police special forces, to be tried on war crimes charges.
* 2001 – Same-sex marriage becomes legal in the Netherlands, the first country to allow it.
* 2002 – The Netherlands legalizes euthanasia, becoming the first nation in the world to do so.
* 2004 – Google introduces its Gmail product to the public. The launch is met with skepticism because of the launch date.
* 2006 – The Serious Organised Crime Agency, dubbed the "British FBI", is created in the United Kingdom.
* 2009 – Croatia and Albania join NATO