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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.
- 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