753 BC – Romulus founds Rome (traditional date).
1898 – Spanish-American War: The United States Navy begins a blockade of Cuban ports. When the U.S. Congress issued a declaration of war on April 25, it declared that a state of war had existed from this date.
1918 – World War I: German fighter ace Manfred von Richthofen, better known as “The Red Baron”, is shot down and killed over Vaux-sur-Somme in France.
1934 – The “Surgeon’s Photograph”, the most famous photo allegedly showing the Loch Ness Monster, is published in the Daily Mail. 65 years later, it is revealed to be a hoax.
1962 – The Seattle World’s Fair (Century 21 Exposition) opens. It is the first World’s Fair in the United States since World War II.
1965 – The 1964-1965 New York World’s Fair opens for its second and final season.
1989 – Tiananmen Square protests of 1989: In Beijing, around 100,000 students gather in Tiananmen Square to commemorate Chinese reform leader Hu Yaobang. Take a look at a very brave man who, weeks later in early June, attempts to stop a column of tanks on their way to Tiananmen Square.
2012 – Two trains are involved in a head-on collision near Sloterdijk, Amsterdam, in the Netherlands, injuring 116 people.
1774 – Jean-Baptiste Biot, French physicist, astronomer, and mathematician (d. 1862)
1838 – John Muir, Scottish-American environmentalist and author (d. 1914)
1926 – Elizabeth II, Queen of the United Kingdom
1935 – Thomas Kean, 48th Governor of New Jersey
1509 – Henry VII of England (b. 1457)
1557 – Petrus Apianus, German mathematician and astronomer (b. 1495)
1910 – Mark Twain, American novelist, humorist, and critic (b. 1835)
From the New York Times of that day:
Danbury, Conn., April 21 — Samuel Langhorne Clemens, “Mark Twain,” died at 22 minutes after 6 tonight. Beside him on the bed lay a beloved book- it was Carlyle’s “French Revolution” -and near the book his glasses, pushed away with a weary sigh a few hours before. Too weak to speak clearly, “Give me my glasses,” he had written on a piece of paper. He had received them, put them down, and sunk into unconsciousness from which he glided almost imperceptibly into death. He was in his seventy-fifth year.
For some time, his daughter Clara and her husband, Ossip Cabrilowitsch, and the humorist’s biographer, Albert Bigelow Paine, had been by the bed waiting for the end, which Drs. Quintard and Halsey had seen to be a matter of minutes. The patient felt absolutely no pain at the end and the moment of his death was scarcely noticeable.
1918 – Manfred von Richthofen, German captain and pilot (b. 1892)
1946 – John Maynard Keynes, English economist and philosopher (b. 1883)
2016 – Prince, American singer-songwriter, guitarist, producer, and actor (b. 1958) It was revealed in this morning’s New York Times that he was 5’2″ with a 22.5 inch waist.
Edited from various sources including historyorb.com, the NYTimes.com Wikipedia and other internet searches