• 1478 – In a conspiracy to replace the Medici family as rulers of the Florentine Republic, the Pazzi family attacked Lorenzo de' Medici and killed his brother Giuliano during High Mass at the Florence Duomo.


• 1777 – American Revolutionary War: Sixteen-year-old Sybil Ludington rode forty miles through the night to warn militiamen under the control of her father that British troops were planning to invade Danbury, Connecticut.

• 1946 – In Naperville, Illinois, US, two passenger trains collided, leaving 45 people dead and some 125 injured.

• 1986 – The Chernobyl Nuclear Power Plant near Chernobyl, Ukrainian SSR, suffered a steam explosion, resulting in a fire, a nuclear meltdown, and the evacuation and resettlement of over 336,000 people around Europe.