| 120301 | October 21 | Event | 1931 – The Sakurakai, a secret society in the Imperial Japanese Army, launches an abortive coup d'état attempt. |
| 120302 | October 21 | Event | 1940 – The first edition of the Ernest Hemingway novel For Whom the Bell Tolls is published. |
| 120303 | October 21 | Event | 1943 – The Provisional Government of Free India is formally declared by Subhas Chandra Bose. |
| 120304 | October 21 | Event | 1944 – World War II: The first kamikaze attack. A Japanese fighter plane carrying a 200-kilogram (440 lb) bomb attacks HMAS Australia off Leyte Island, as the Battle of Leyte Gulf began. |
| 120305 | October 21 | Event | 1944 – World War II: Nemmersdorf massacre against the German civilians takes place. |
| 120306 | October 21 | Event | 1944 – World War II: Battle of Aachen: The city of Aachen falls to American forces after three weeks of fighting, making it the first German city to fall to the Allies. |
| 120307 | October 21 | Event | 1945 – Women's suffrage: Women are allowed to vote in France for the first time. |
| 120308 | October 21 | Event | 1950 – Korean War: Heavy fighting begins between British and Australian forces from the 27th British Commonwealth Brigade and the North Korean 239th Regiment during the Battle of Yongju. |
| 120309 | October 21 | Event | 1956 – Mau Mau Uprising: Kenyan rebel leader Dedan Kimathi is captured by the British Army, signalling the ultimate defeat of the rebellion, and essentially ending the British military campaign. |
| 120310 | October 21 | Event | 1959 – In New York City, the Solomon R. Guggenheim Museum, designed by Frank Lloyd Wright, opens to the public. |
| 120311 | October 21 | Event | 1959 – U.S. President Dwight D. Eisenhower signs an executive order transferring Wernher von Braun and other German scientists from the United States Army to NASA. |
| 120312 | October 21 | Event | 1965 – Comet Ikeya–Seki approaches perihelion, passing 450,000 kilometers (279,617 miles) from the sun. |
| 120313 | October 21 | Event | 1966 – Aberfan disaster: A slag heap collapses on the village of Aberfan in Wales, killing 144 people, mostly schoolchildren. |
| 120314 | October 21 | Event | 1967 – Vietnam War: More than 100,000 war protesters gather in Washington, D.C.. A peaceful rally at the Lincoln Memorial is followed by a march to The Pentagon and clashes with soldiers and United States Marshals protecting the facility. Similar demonstr |
| 120315 | October 21 | Event | 1969 – A coup d'état in Somalia brings Siad Barre to power and establishes a socialist republic in Somalia. |
| 120316 | October 21 | Event | 1971 – A gas explosion kills 22 people at a shopping center in Clarkston, East Renfrewshire, near Glasgow, Scotland. |
| 120317 | October 21 | Event | 1973 – John Paul Getty III's ear is cut off by his kidnappers and sent to a newspaper in Rome it doesn't arrive until November 8. |
| 120318 | October 21 | Event | 1973 – Fred Dryer of the then Los Angeles Rams becomes the first player in NFL history to score two safeties in the same game. |
| 120319 | October 21 | Event | 1977 – The European Patent Institute is founded. |
| 120320 | October 21 | Event | 1978 – Australian civilian pilot Frederick Valentich vanishes in a Cessna 182 over the Bass Strait south of Melbourne, after reporting contact with an unidentified aircraft. |
| 120321 | October 21 | Event | 1979 – Moshe Dayan resigns from the Israeli government because of strong disagreements with Prime Minister Menachem Begin over policy towards the Arabs. |
| 120322 | October 21 | Event | 1981 – Andreas Papandreou becomes Prime Minister of Greece, ending an almost 50-year-long system of power dominated by conservative forces. |
| 120323 | October 21 | Event | 1983 – The metre is defined at the seventeenth General Conference on Weights and Measures as the distance light travels in a vacuum in 1/299,792,458 of a second. |
| 120324 | October 21 | Event | 1986 – In Lebanon, pro-Iran kidnappers claim to have abducted American writer Edward Tracy (he is released in August 1991). |
| 120325 | October 21 | Event | 1987 – Jaffna hospital massacre is carried out by Indian Peace Keeping Force in Sri Lanka killing 70 ethnic Tamil patients, doctors and nurses. |
| 120326 | October 21 | Event | 1994 – North Korea nuclear weapons program: North Korea and the United States sign an agreement that requires North Korea to stop its nuclear weapons program and agree to inspections. |
| 120327 | October 21 | Event | 1994 – In Seoul, 32 people are killed when the Seongsu Bridge collapses. |
| 120328 | October 21 | Event | 2005 – Images of the dwarf planet Eris are taken and subsequently used in documenting its discovery by the team of Michael E. Brown, Chad Trujillo, and David L. Rabinowitz. |
| 120329 | October 21 | Event | 2012 – A shooting at a spa in Brookfield, Wisconsin, leaves four people dead, including the shooter. |
| 120330 | October 21 | Event | 2013 – Record smog closes schools, roadways, and the airport in Harbin, China. |