Here's a few to get things started:
1865, John Wilkes Booth shot President Lincoln;
1912, RMS Titanic struck its iceberg... it sank on April 15;
1918, two American pilots engage in America's first aerial dogfight with enemy aircraft at Toul, France;
1986, the US launched air strikes against Libya in retaliation for the Libyan sponsorship of terrorism against American troops and citizens.