April 21 is the 111th day of the year in the Gregorian calendar (112th in leap years).
There are 254 days remaining.
- 753 BC - Romulus founds Rome (traditional)
- 1836 - Battle of San Jacinto: Texas forces under Sam Houston defeat and capture Antonio Lopez de Santa Anna
- 1863 - Bahá'u'lláh is proclaimed as the head of the Baha'i Faith
- 1898 - Spanish-American War: The United States Congress, on April 25, recognized that a state of war had existed between the United States and Spain as of this date
- 1912 - The New York Giants and New York Yankees play an exhibition game to benefit survivors of the Titanic.
- 1953 - Roy Cohn[?] and David Schine[?], two of Senator Joseph McCarthy's chief aides, recommend the removal of 30,000 books from the libraries of the United States Information Service[?] posts in Europe, including works by Dashiell Hammett, W. E. B. Du Bois, Herman Melville, John Steinbeck and Henry David Thoreau, calling them "pro-Communist"
- 1970 - Hutt River Province secedes from the Commonwealth of Australia
- 1975 - Vietnam War: President of South Vietnam Nguyen Van Thieu flees Saigon, as Xuan Loc[?], the last South Vietnamese outpost blocking a direct North Vietnamese assault on Saigon, falls
- 1994 - The first discovery of extrasolar[?] planets are announced by astronomer Alexander Wolszczan[?].
- 1729 - Tsarina Catherine II of Russia (+ 1796)
- 1816 - Charlotte Brontë, author (+ 1855)
- 1838 - John Muir, environmentalist (+ 1914)
- 1851 - Charles Barrois, geologist (+ 1939)
- 1864 - Max Weber, economist and sociologist (+ 1920)
- 1912 - Marcel Camus, film director
- 1914 - Anthony Quinn, actor (+ 2001)
- 1924 - Clara Ward[?], gospel music singer (+ 1973)
- 1926 - Queen Elizabeth II of the United Kingdom
- 1929 - Martin Kruse[?], theologian
- 1932 - Elaine May[?], comedienne
- 1947 - Iggy Pop
- 1951 - Michael Hartley Freedman[?], mathematician.
- 1958 - Andie MacDowell, actress
- 1959 - Robert Smith, musician
- 1699 - Jean Racine, dramatist (* 1639)
- 1793 - John Michell[?], seismologist (* 1724)
- 1910 - Mark Twain, author, humorist (* 1835)
- 1918 - Manfred von Richthofen, pilot (* 1892)
- 1946 - John Maynard Keynes, economist (* 1883)
- 1956 - Charles MacArthur, writer (* 1895)
- 1978 - Thomas Wyatt Turner[?], civil rights advocate and agricultural engineer (* 1877)
- 1980 - Aleksandr Oparin[?], biochemist (* 1894)
- 1985 - Rudi Gernreich[?], fashion designer
- 1999 - Charles 'Buddy' Rogers[?], actor, musician
- 2001 - Jack Haley Jr.[?], producer
- 2003 - Nina Simone, singer and pianist (* 1933)
April 20 - April 22 - March 21 - May 21 -- listing of all days
January, February, March, April, May, June, July,
August, September, October, November, December