- Apr 5 The Greco-Turkish War, also called "Thirty Days' War", is declared between Greece and the Ottoman Empire.
- Apr 19 1st Boston Marathon (B.A.A. Road Race), won by John J. McDermott in 2:55:10; the world's oldest annual marathon inspired by success of the first marathon at the 1896 Summer Olympics
- Apr 22 NYC Jewish newspaper "Forward" begins publishing (still active)
Discovery of the Electron
Apr 30 English physicist and head of the Cavendish laboratory J. J. Thomson announces his discovery of the electron in a lecture to the Royal Institution
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