April 20
| 1139 | The Second Lateran Council opens in Rome. | |
| 1657 | English Admiral Robert Blake fights his last battle when he destroys the Spanish fleet in Santa Cruz Bay. | |
| 1769 | Ottawa Chief Pontiac is murdered by an Indian in Cahokia. | |
| 1770 | Captain Cook discovers Australia. | |
| 1775 | British troops begin the siege of Boston. | |
| 1792 | France declares war on Austria, Prussia, and Sardinia. | |
| 1809 | Napoleon defeats Austria at Battle of Abensberg, Bavaria. | |
| 1836 | The Territory of Wisconsin is created. | |
| 1841 | Edgar Allen Poe's first detective story is published. | |
| 1861 | Robert E. Lee resigns from the U.S. Army. | |
| 1879 | The first mobile home (horse-drawn) is used in a journey from London to Cyprus. | |
| 1916 | Wrigley Field opens in Chicago. | |
| 1919 | The Polish Army captures Vilno, Lithuania from the Soviets. | |
| 1940 | The first electron microscope is demonstrated. | |
| 1942 | Pierre Laval, the premier of Vichy France, in a radio broadcast, establishes a policy of "true reconciliation with Germany." | |
| 1945 | Soviet troops begin their attack on Berlin. | |
| 1951 | General MacArthur addresses a joint session of Congress after being relieved by President Truman. | |
| 1953 | Operation Little Switch begins in Korea, the exchange of sick and wounded prisoners of war. | |
| 1962 | The New Orleans Citizens Committee gives free one-way ride to blacks to move North. | |
| 1967 | U.S. planes bomb Haiphong for first time during the Vietnam War. | |
| 1999 | Two students enter Columbine High School in Littleton, Colorado and open fire with multiple firearms, killing 13 students and teachers, wounding 25 and eventually shooting themselves. | |
| Born on April 20 | ||
| 121 | Marcus Aurelius, 16th Roman emperor, philosopher. | |
| 1745 | Philippe Pinel, founder of psychiatry. | |
| 1807 | Aloysius Bertrand ("Gaspard de la Nuit"), French poet. | |
| 1808 | Louis-Napoleon (Napoleon III), emperor of France. | |
| 1850 | Daniel Chester French, sculptor. | |
| 1889 | Adolf Hitler, Facist dictator of Nazi Germany (1933-1945). | |
| 1893 | Harold Lloyd, film comedian. | |
| 1893 | Joan Miró, Spanish painter. | |
| 1927 | Alex Muller, Nobel Prize-winning physicist. | |
