This pretty much says it all (4/20 or 20/4 you filthy metric apes ):
Events:
1653 - Oliver Cromwell grows sick with a broken pelvis.
1689 - The former King James II of England, now deposed, lays siege to Derry.
1770 - Lieutenant James Cook's expedition (first voyage) makes first sighting of eastern Australian coastline, naming the spot Cape Hicks. His logbook recorded the date as April 19, but the 20th was the actual calendar date.
1775 - American Revolutionary War: British troops begin siege of Boston, Massachusetts
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.
1862 - The first pasteurization test completed by Louis Pasteur and Claude Bernard.
1902 - Pierre and Marie Curie refine radium chloride.
1918 - Manfred von Richthofen, aka The Red Baron, shoots down his 79th and 80th victims marking his final victories before his death the following day.
1972 - Apollo 16 lands on the Moon.
1999 - Eric Harris and Dylan Klebold kill 12 students and a teacher before turning their guns on themselves in the Columbine High School massacre.
Births:
570 - Mohammed, founder of Islam was born in Mecca (appx. year)
1889 - Adolf Hitler, Austrian-born German dictator
1964 - Crispin Glover, American actor
Carmen Electra, American actress
Deaths:
1769 - Pontiac, Chief of the Ottawa
1912 - Bram Stoker, Irish author
1992 - Benny Hill, British comedian
Holidays and observances:
2003, 2014 - Easter Sunday
April 20 (4/20) is associated with 420 (drug culture)
Astrology: First day of sun sign Taurus