April 16, 1862: Slavery abolished in District of Columbia

April 17, 1492: Christopher Columbus signs a contract with the Spanish monarchs to find the "Indies" with the stated goal of converting people to Catholicism. This promises him 10% of all riches found, and the governorship of any lands encountered.

April 18, 1906: San Francisco earthquake and fire kills nearly 4,000 while destroying 75% of the city.

April 19, 1782: John Adams secures Dutch Republic's recognition of the United States as an independent government and house he purchased in The Hague, Netherlands became first American embassy.

April 20, 1736: French mathematician Pierre Louis Maupertuis begins Lapland expedition to measure latitude and shape of the earth, joined by fellow scientists Anders Celsius, Charles Etienne Louis Camus, Alexis Clairaut, and Pierre-Charles Le Monnier.

April 21, 753 BC: Romulus and Remus found Rome (traditional date)

April 22, 1969: First human eye transplant performed.

April 23, 1920: The national council in Turkey denounces the government of Sultan Mehmed VI and announces a temporary constitution.

April 24, 1884: National Medical Association of Black physicians organizes in Atlanta

April 25, 1507: German cartographer Martin Waldseemüller first to use the name America on his world map "Universalis Cosmographia"

April 26, 1956: First modern container ship, the Ideal X, leaves Port Newark, New Jersey for Houston, Texas

April 27, 1509: Pope Jullius II excommunicates Italian state of Venice

April 28, 1611: Establishment of the Pontifical and Royal University of Santo Tomas, The Catholic University of the Philippines, oldest existing university in Asia and largest Catholic university in the world

April 29, 1916: Irish republicans abandon the post office in Dublin and surrender unconditionally, marking the end of the Easter Rising

April 30, 711: Islamic conquest of Iberia: Moorish troops led by Tariq ibn-Ziyad land at Gibraltar to begin their invasion of the Iberian Peninsula (Al-Andalus).