Ascension Thursday (Western Christianity, 2008); Yom HaShoah in Israel (2008); May Day; International Workers' Day; Beltane in Ireland and Scotland

• 1707 – The Kingdoms of England and Scotland merged to form the Kingdom of Great Britain, a single kingdom encompassing the entire island of Great Britain with a single parliament and government based in Westminster.
• 1840 – The United Kingdom of Great Britain and Ireland first issued the Penny Black (pictured), the first official adhesive postage stamp.
• 1893 – The World's Columbian Exposition, a World's Fair to celebrate the 400th anniversary of Christopher Columbus' discovery of the New World, opened in Chicago.
• 1898 – The American Asiatic Squadron under Commodore George Dewey defeated the Spanish Pacific Squadron under Admiral Patricio Montojo at the Battle of Manila Bay, the first engagement of the Spanish-American War.
• 1956 – A doctor in Japan reported an "epidemic of an unknown disease of the central nervous system", marking the official discovery of Minamata disease.