April 11, 1976: The Apple I is created. To finance its creation, Steve Jobs sold his only motorized means of transportation, a VW Microbus, for a few hundred dollars, and Steve Wozniak sold his HP-65 calculator for $500.
April 12, 1908: The Great Chelsea Fire of 1908 burns more than a third of this Massachusetts Town. 19 lives are lost.
April 12, 1961: The Soviet cosmonaut Yuri Gagarin becomes the first human to travel into outer space and perform the first manned orbital flight, Vostok 1.
April 13, 1918: 40 patients are killed in a fire at at the Central Oklahoma State Mental Hospital. Only two of the victims are identified and the rest are buried in an unmarked mass grave.
April 13, 1964: At the Academy Awards, Sidney Poitier becomes the first African-American male to win the Best Actor award for the 1963 film Lilies of the Field.
April 14, 1749: The HMS *Namur *sinks during a storm killing 520.
April 14, 1561: Around dawn, residents of Nuremberg, Germany see what they describe as an aerial battle, followed by the appearance of a large black triangular object and then a large crash outside of the city. According to witnesses, there are hundreds of spheres, cylinders and other odd-shaped objects that move erratically overhead. The phenomenon has been interpreted by some modern UFO enthusiasts as an aerial battle of extraterrestrial origin.
April 14, 1865: John Wilkes Booth, an actor and Confederate sympathizer, fatally shoots President Abraham Lincoln at a play at Ford’s Theater in Washington, D.C.
“Other than that, Mrs. Lincoln, how was the play?” 
April 15, 1912: The RMS *Titanic *sinks after hitting an iceberg. Over 1500 die.
April 15, 2013: Two bombs explode near the finish line at the Boston Marathon, killing three people and injuring 264 others.
**April 16, 1947 **: The Texas City Disaster occurs. This is America’s deadliest industrial disaster. Officially the death toll is 581, but the real total is probably much higher. 2200 tons of ammonium nitrate aboard the SS Grandcamp explode, killing hundreds of onlookers who had come to the dock to see the burning ship. Many others died up to 5 miles away from the blast. Thousands were injured.Here is a good short video about the event.
April 16, 1943: Albert Hofmann accidentally discovers the hallucinogenic effects of the research drug LSD. He intentionally takes the drug three days later on April 19.
April 17, 2013: The West Texas Fertilizer Explosionoccurs. 15 people are killed, over 160 injured, and hundreds of homes are destroyed when ammonium nitrate fertlizer catches fire at the West Fertilizer Company in West, Texas. Here is a good short video about the event.
April 17, 1961: A group of Cuban exiles financed and trained by the CIA lands at the Bay of Pigs in Cuba with the aim of ousting Fidel Castro. It doesn’t end well.
April 17, 1982: Patriation of the Constitution of Canada: Her Majesty and Prime Minister Trudeau sign the Proclamation bringing the Constitution Act, 1982 into effect. Britain can no longer legislate for Canada. Jean Chrétien, the Minister of Justice, also signs the proclamation, but has a problem with his pen and whispers “merde” under his breath. Her Majesty hears it and smiles.
April 18, 1906: The Great San Francisco Earthquakekills about 3000 and destroys 80% of the city.
April 18, 1923: The original Yankee Stadium, “The House that Ruth Built”, opens.
April 20, 1944: The SS *Paul Hamilton * is hit by an aerial torpedo from a German bomber and blows up in the Mediterranean Sea. The Liberty ship, loaded with explosives and men, is lost with all hands. Of the 580 aboard, only one body is ever recovered.
April 20, 1912: Opening day for Fenway Park, to this day the home of baseball’s Boston Red Sox.
April 20, 1916: Opening day for what is now known as Wrigley Field, home of the Chicago Cubs.
April 20, 1999: Eric Harris and Dylan Klebold killed 13 people and injured 24 others before committing suicide at Columbine High School in Columbine, Colorado.