April 20 is the 110th day of the year. There are 255 days remaining until the end of the year. This date is slightly more likely to fall on a Monday, Wednesday or Friday (58 in 400 years each) than on Saturday or Sunday (57), and slightly less likely to occur on a Tuesday or Thursday (56).
+ 1789 – George Washington arrives at Grays Ferry, Philadelphia while en route to Manhattan for his inauguration
+ 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.
+ 1912 – Opening day for baseball's Tiger Stadium in Detroit, and Fenway Park in Boston.
+ 1916 – The Chicago Cubs play their first game at Weeghman Park (currently Wrigley Field), defeating the Cincinnati Reds 7–6 in 11 innings.
+ 1999 – Columbine High School massacre: Eric Harris and Dylan Klebold killed 13 people and injured 24 others before committing suicide at Columbine High School in Columbine, Colorado.
+ 2007 – Johnson Space Center shooting: William Phillips with a handgun barricades himself in NASA's Johnson Space Center in Houston, Texas before killing a male hostage and himself.
+ 2010 – The Deepwater Horizon drilling rig explodes in the Gulf of Mexico, killing eleven workers and beginning an oil spill that would last six months.
+ 1941 – Ryan O'Neal, American actor
+ 1949 – Jessica Lange, American actress
+ 1951 – Luther Vandross, American singer-songwriter and producer (d. 2005)
+ 1961 – Don Mattingly, American baseball player, coach, and manager
+ 1976 – Joey Lawrence, American actor, game show host, and singer