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1921 (MCMXXI) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Friday of the Julian calendar, the 1921st year of the Common Era (CE) and Anno Domini (AD) designations, the 921st year of the 2nd millennium, the 21st year of the 20th century, and the 2nd year of the 1920s decade. As of the start of 1921, the Gregorian calendar was 13 days ahead of the Julian calendar, which remained in localized use until 1923.
- May 1–7 – Jaffa riots: Riots at Jaffa, Mandatory Palestine result in 47 Jewish and 48 Arab deaths.
- May 2–July 5 – Third Silesian Uprising: Poles in Upper Silesia rise against the Germans.
- May 3 – The province of Northern Ireland is created within the United Kingdom.
- May 5
- May 6 – The German-Russian Provisional Agreement is signed; Germany recognises the Soviet regime in Russia.
- May 14–15 – The major May 1921 geomagnetic storm occurs.
- May 14–17 – Violent anti-European riots occur in Cairo and Alexandria, Egypt.
- May 16 – The Communist Party of Czechoslovakia is founded.
- May 19 – The Emergency Quota Act is passed by the United States Congress, establishing national quotas on immigration. Because this drastically limits immigration from Eastern Europe, Jews emigrating from there begin to prefer Palestine as a destination rather than the U.S.
- May 22 – In the first golf international between the two countries, the United States beats the United Kingdom 9 rounds to 3.
- May 23–July 16 – The Leipzig War Crimes Trials are held in Germany.
- May 24 – 1921 Irish elections: In the first Northern Ireland general election for the new Parliament of Northern Ireland, Ulster Unionists win 40 out of 52 seats. The dominant-party system here will last for fifty years.
- May 25 – Irish War of Independence: The Irish Republican Army occupies and burns The Custom House in Dublin, the centre of local government in Ireland. Five IRA men are killed, and over 80 are captured by the British Army which surrounds the building.
- May 26 – A general strike begins in Norway.
- May 31–June 1 – Tulsa Race Riot (Greenwood Massacre): Mobs of white residents attack black residents and businesses in Greenwood District, Tulsa, Oklahoma. The official death toll is 36, but later investigations suggest an actual figure between 100 and 300. 1,250 homes are destroyed and roughly 6,000 African Americans imprisoned in one of the worst incidents of mass racial violence in the United States.
- July 1
- July 2 – U.S. President Warren Harding signs a joint congressional resolution, declaring an end to America's state of war with Germany, Austria and Hungary.
- July 4 – A new conservative government is formed in Italy, by Ivanoe Bonomi.
- July 11
- July 14 – A Massachusetts jury finds Nicola Sacco and Bartolomeo Vanzetti guilty of first degree murder, following a widely publicized trial.
- July 17 – The Republic of Mirdita is proclaimed near the Albanian-Serbian border, with Yugoslav support.
- July 18 – The first BCG vaccination against tuberculosis is given.
- July 21 – Rif War – Battle of Annual: Spanish troops are dealt a crushing defeat, at the hands of Abd el-Krim.
- July 22 – The Anglo-Irish truce, agreed 10 days earlier, is officially declared in London.
- July 23 – The Communist Party of China (CPC) launches the first founding National Congress.
- July 26 – U.S. President Warren G. Harding receives Princess Fatima of Afghanistan, and Stanley Clifford Weyman.
- July 27 – Researchers at the University of Toronto, led by biochemist Frederick Banting, announce the discovery of the hormone insulin.
- July 29 – Adolf Hitler becomes Führer of the Nazi Party.
беларуская (тарашкевіца): 1921
srpskohrvatski / српскохрватски: 1921