History of the Day for:
April 20
- 1775: American Revolutionary War: the Siege of Boston begins, following the battles at Lexington and Concord.
- 1812: The fourth vice president of the United States, George Clinton, died in Washington at age 73, becoming the first vice president to die while in office.
- 1832: Congress established the first U.S. national park, Hot Springs National Park.
- 1836: Congress established the Territory of Wisconsin.
- 1889: Adolf Hitler was born.
- 1898: President McKinley signed a congressional resolution that was passed the day before recognizing Cuban independence from Spain and authorizing U.S. military intervention to achieve that goal.
- 1902: Marie and Pierre Curie isolated the radioactive element radium.
- 1934: Shirley Temple debuted in "Stand Up And Cheer."
- 1935: "Your Hit Parade" was first broadcast on radio.
- 1940: RCA demonstrated its new electron microscope in Philadelphia.
- 1945: The U.S. Seventh Army and allied forces captured Nuremberg and Stuttgart in Germany.
- 1961: The Federal Communications Commission gave approval for FM stereo broadcasting.
- 1971: The Supreme Court upheld busing as a means of achieving racial desegregation in schools.
- 1972: The manned lunar module from "Apollo 16" landed on the moon.
- 1978: Korean Air Flight 902 is shot down by Soviets.
- 1980: Climax of Berber Spring in Algeria as hundreds of Berber political activists are arrested.
- 1985: ATF raid on The Covenant, The Sword, and the Arm of the Lord compound in northern Arkansas.
- 1986: Pianist Vladimir Horowitz performs in his native Russia for the first time in 61 years.
- 1986: Professional basketball player Michael Jordan sets all-time record for points in an NBA playoff game with 63 against the Boston Celtics.
- 1998: German terrorist group Red Army Faction announces their dissolution after 28 years.
- 1999: Columbine High School massacre: Eric Harris and Dylan Klebold kill 13 people and injure 24 others before committing suicide at Columbine High School in Jefferson County, Colorado.
- 2001: China removes homosexuality from its list of mental illnesses.
- 2007: Johnson Space Center Shooting: A man with a handgun barricades himself in NASA's Johnson Space Center in Houston, Texas before killing a male hostage and himself.
- 2008: Danica Patrick wins the Indy Japan 300 becoming the first female driver in history to win an Indy car race.