| 45481 | April 20 | Event | 1775 – American Revolutionary War: The Siege of Boston begins, following the battles at Lexington and Concord. |
| 45482 | April 20 | Event | 1789 – George Washington arrives at Grays Ferry, Philadelphia while en route to Manhattan for his inauguration |
| 45483 | April 20 | Event | 1792 – France declares war against the "King of Hungary and Bohemia", the beginning of French Revolutionary Wars. |
| 45484 | April 20 | Event | 1800 – The Septinsular Republic is established. |
| 45485 | April 20 | Event | 1809 – Two Austrian army corps in Bavaria are defeated by a First French Empire army led by Napoleon at the Battle of Abensberg on the second day of a four-day campaign that ended in a French victory. |
| 45486 | April 20 | Event | 1810 – The Governor of Caracas declares independence from Spain. |
| 45487 | April 20 | Event | 1818 – The case of Ashford v Thornton ends, with Abraham Thornton allowed to go free rather than face a retrial for murder, after his demand for trial by battle is upheld. |
| 45488 | April 20 | Event | 1828 – René Caillié becomes the first non-Muslim to enter Timbuktu. |
| 45489 | April 20 | Event | 1836 – U.S. Congress passes an act creating the Wisconsin Territory. |
| 45490 | April 20 | Event | 1861 – American Civil War: Robert E. Lee resigns his commission in the United States Army in order to command the forces of the state of Virginia. |
| 45491 | April 20 | Event | 1862 – Louis Pasteur and Claude Bernard complete the experiment falsifying the theory of spontaneous generation. |
| 45492 | April 20 | Event | 1865 – Astronomer Angelo Secchi demonstrates the Secchi disk, which measures water clarity, aboard Pope Pius IX's yacht, the L'Immaculata Concezion. |
| 45493 | April 20 | Event | 1871 – The Civil Rights Act of 1871 becomes law. |
| 45494 | April 20 | Event | 1876 – The April Uprising begins. Its suppression shocks European opinion, and Bulgarian independence becomes a condition for ending the Russo-Turkish War. |
| 45495 | April 20 | Event | 1884 – Pope Leo XIII publishes the encyclical Humanum genus. |
| 45496 | April 20 | Event | 1902 – Pierre and Marie Curie refine radium chloride. |
| 45497 | April 20 | Event | 1908 – Opening day of competition in the New South Wales Rugby League. |
| 45498 | April 20 | Event | 1912 – Opening day for baseball's Tiger Stadium in Detroit, and Fenway Park in Boston. |
| 45499 | April 20 | Event | 1914 – Nineteen men, women, and children die in the Ludlow Massacre during a Colorado coal-miner's strike. |
| 45500 | April 20 | Event | 1916 – The Chicago Cubs play their first game at Weeghman Park (currently Wrigley Field), defeating the Cincinnati Reds 7–6 in 11 innings. |
| 45501 | April 20 | Event | 1918 – Manfred von Richthofen, a.k.a. The Red Baron, shoots down his 79th and 80th victims, his final victories before his death the following day. |
| 45502 | April 20 | Event | 1922 – The Soviet government creates South Ossetian Autonomous Oblast within Georgian SSR. |
| 45503 | April 20 | Event | 1926 – Western Electric and Warner Bros. announce Vitaphone, a process to add sound to film. |
| 45504 | April 20 | Event | 1939 – Adolf Hitler's 50th birthday is celebrated as a national holiday in Nazi Germany. |
| 45505 | April 20 | Event | 1939 – Billie Holiday records the first civil rights song "Strange Fruit". |
| 45506 | April 20 | Event | 1945 – World War II: US troops capture Leipzig, Germany, only to later cede the city to the Soviet Union. |
| 45507 | April 20 | Event | 1945 – World War II: Führerbunker: Adolf Hitler makes his last trip to the surface to award Iron Crosses to boy soldiers of the Hitler Youth. |
| 45508 | April 20 | Event | 1945 – Twenty Jewish children used in medical experiments at Neuengamme are killed in the basement of the Bullenhuser Damm school. |
| 45509 | April 20 | Event | 1946 – The League of Nations officially dissolves, giving most of its power to the United Nations. |
| 45510 | April 20 | Event | 1951 – Dan Gavriliu performs the first surgical replacement of a human organ. |