October 21 is the 294th day of the year (295th in leap years) in the Gregorian Calendar, with 71 days remaining.
- 686 - Conon becomes Pope.
- 1600 - Tokugawa Ieyasu defeats the leaders of rival Japanese clans in the Battle of Sekigahara, which marks the beginning of the Tokugawa shogunate, who in effect rule Japan until the mid-Nineteenth century.
- 1797 - In Boston Harbor, the 44-gun United States Navy frigate USS Constitution is launched.
- 1805 - Napoleonic Wars: Battle of Trafalgar - a British fleet led by Admiral Lord Nelson defeats a combined French and Spanish fleet off the coast of Spain.
- 1824 - Joseph Aspdin patents Portland cement.
- 1854 - Florence Nightingale and a staff of 38 nurses were sent to the Crimean War.
- 1861 - American Civil War: Battle of Ball's Bluff - Union forces under Colonel Edward Baker are defeated by Confederate troops in the second major battle of the war. Baker, a close friend of Abraham Lincoln, is killed in the fighting.
- 1867 - 'Manifest Destiny': Medicine Lodge Treaty - Near Medicine Lodge Creek, Kansas a landmark treaty is signed by southern Great Plains Indian leaders. The treaty requires Native American Plains tribes to relocate a reservation in western Oklahoma.
- 1879 - Using a filament of carbonized thread, Thomas Edison tests the first practical electric light bulb (it lasted 13 1/2 hours before burning out).
- 1895 - The Republic of Taiwan collapses as Japanese forces invade.
- 1902 - In the United States, a five month strike by United Mine Workers ends.
- 1941 - World War II: Germans rampage in Yugoslavia, killing thousands of civilians.
- 1944 - The first kamikaze attack: HMAS Australia was hit by a Japanese plane carrying a 200 kg (441 pound) bomb off Leyte Island, as the Battle of Leyte Gulf began.
- 1945 - Women's suffrage: Women are allowed to vote in France for the first time.
- 1945 - Argentine military officer and politician Juan Perón married actress Evita.
- 1947 - 21 die as a fire destroys an asylum in Hoff, Germany.
- 1957 - The movie Jailhouse Rock, starring Elvis Presley, opens.
- 1959 - In New York City, the Solomon R. Guggenheim Museum opens to the public. It was designed by Frank Lloyd Wright.
- 1959 - US President Dwight D. Eisenhower signs an executive order transferring Wernher Von Braun and other German scientists from the United States Army to NASA.
- 1966 - Aberfan disaster: A coal tip falls on the village of Aberfan in Wales, killing 144 people, mostly schoolchildren
- 1967 - Vietnam War: More than 100,000 war protesters gather in Washington, DC. 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 (event lasts until October 23; 683 people will be arrested). Similar demonstrations occurred simultaneously in Japan and Western Europe.
- 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.
- 1980 - 1980 World Series: In 6 games, the Philadelphia Phillies win their first World Series.
- 1986 - In Lebanon, pro-Iranian kidnappers claim to have abducted American writer Edward Tracy (he will be released in August 1991).
- 1987 - Former Miss America Bess Myerson is arrested on charges of bribery, conspiracy, and mail fraud, all involving an alimony-fixing scandal. She is later found not guilty.
- 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.
- 1997 - Hotel owners from the Detroit area meet to discuss Jack Kevorkian's practice of leaving corpses in hotel rooms.
- 1997 - The government of Singapore announces in a widely-publicized "toilet alert" that the drive for toilet cleanliness is a great success; five toilets were selected by citizens as toilet role models.
- 1449 - George, Duke of Clarence
- 1660 - Georg Ernst Stahl, German scientist
- 1772 - Samuel Taylor Coleridge, British poet (d. 1834)
- 1821 - The Monster of Glamis (d. unknown)
- 1833 - Alfred Nobel, Swedish inventor and benefactor of the Nobel Prize (d. 1896)
- 1895 - Edna Purviance, actress (d. 1958)
- 1912 - Sir Georg Solti, conductor (d. 1997)
- 1914 - Martin Gardner, American writer on mathematics and games
- 1917 - Dizzy Gillespie, American jazz musician (d. 1993)
- 1920 - Hy Averback, film and television director (d. 1997)
- 1921 - Malcolm Arnold, British composer
- 1924 - Celia Cruz, singer (d. 2003)
- 1928 - Whitey Ford, American baseball player
- 1929 - Ursula K. Le Guin, American science fiction author
- 1940 - Manfred Mann, musician
- 1941 - Steve Cropper, musician
- 1942 - Elvin Bishop, musician
- 1942 - Judge Judy Sheindlin, American judge, television host
- 1943 - Brian Piccolo, American football star (d. 1970)
- 1949 - Benjamin Netanyahu, the 9th Prime Minister of Israel
- 1955 - Rich Mullins, American musician (d. 1997)
- 1956 - Carrie Fisher, actress, writer
- 1972 - Felicity Andersen, actress
- 310 - Pope Eusebius
- 1558 - Julius Caesar Scaliger, humanist scholar
- 1805 - Horatio Nelson, British admiral
- 1896 - James Henry Greathead, British engineer
- 1931 - Arthur Schnitzler, Austrian writer
- 1944 - Alois Kayser, German missionary, working in Nauru
- 1969 - Waclaw Sierpinski, Polish mathematician
- 1969 - Jack Kerouac, American beat novelist
- 1975 - Charles Reidpath, American athlete
- 1984 - François Truffaut, French film director
- 1986 - Lionel Murphy, Australian Labor Party politician and High Court judge.
- 1995 - Shannon Hoon, lead singer of pop band Blind Melon
- 2003 - Elliott Smith, musician
October 20 - October 22 - November 21 - September 21 - more historical anniversaries
es:21 de octubre
eo:21-a de oktobro
pt:21 de Outubro