May 17

Wallachian forces defeat an invading Ottoman army at the Battle of Rovine.

Wallachian voivode Mircea the Elder led his forces to a defensive victory over an invading Ottoman army at the Battle of Rovine, one of several successful early Wallachian resistances against Ottoman expansion that delayed, though ultimately did not prevent, the region's eventual subjugation as an Ottoman tributary state in later centuries.

French colonist Paul de Chomedey founds Ville-Marie, the settlement that grew into the city of Montreal.

French colonist Paul de Chomedey, Sieur de Maisonneuve, founded the settlement of Ville-Marie on the island of Montreal despite warnings that the site was too exposed to attack, establishing what began as a small Catholic mission outpost dedicated to converting Indigenous peoples and grew over subsequent centuries into one of Canada's largest and most culturally significant cities.

Great Britain declares war on France, formally beginning the Seven Years' War, a conflict that spanned multiple continents.

Great Britain formally declared war on France, opening the sprawling global conflict known as the Seven Years' War, fought simultaneously across battlefields in Europe, North America, the Caribbean, West Africa, and India, and later described by Prime Minister Winston Churchill as arguably the first true world war in history.

Twenty-four New York merchants sign the Buttonwood Agreement, founding what became the New York Stock Exchange.

Twenty-four New York City stockbrokers and merchants signed the Buttonwood Agreement while gathered beneath a buttonwood tree on Wall Street, establishing foundational rules governing commission rates and trading practices among themselves that evolved over subsequent decades into the New York Stock Exchange, now one of the largest and most influential financial markets in the world.

Members of the Melbourne Football Club codify the first rules of Australian rules football.

Members of the Melbourne Football Club, seeking to formalize a distinctly Australian winter sport suited to the colony's hard grounds, drafted the first written rules for what became Australian rules football, a fast-paced game blending elements of rugby and Gaelic football that remains one of Australia's most popular and passionately followed spectator sports.

L. Frank Baum's "The Wonderful Wizard of Oz" is first published in the United States.

L. Frank Baum's "The Wonderful Wizard of Oz," illustrated by W. W. Denslow, was first published in the United States and quickly became a bestseller, introducing characters like Dorothy, the Scarecrow, and the Tin Man that became deeply embedded in American popular culture through numerous later stage and film adaptations, most famously the beloved 1939 MGM movie.

The U.S. Supreme Court rules unanimously in "Brown v. Board of Education" that racial segregation in public schools is unconstitutional.

The U.S. Supreme Court issued its landmark unanimous ruling in "Brown v. Board of Education," declaring state-mandated racial segregation in public schools inherently unconstitutional and formally overturning the decades-old "separate but equal" doctrine established by "Plessy v. Ferguson," a pivotal legal victory that energized the broader American civil rights movement.

The World Health Organization removes homosexuality from its official classification of mental disorders.

The World Health Organization formally removed homosexuality from its International Classification of Diseases, a significant milestone in the global effort to depathologize sexual orientation after decades in which it had been officially classified alongside mental illnesses. The anniversary is now marked internationally each year as the International Day Against Homophobia, Biphobia, and Transphobia.

Massachusetts becomes the first U.S. state to legally perform same-sex marriages.

Massachusetts became the first U.S. state to legally perform same-sex marriages following a landmark 2003 state supreme court ruling that found denying marriage rights to same-sex couples unconstitutional, a pioneering moment in the American legal and cultural battle over marriage equality that preceded the U.S. Supreme Court's nationwide legalization by more than a decade.

Trains cross the border between North and South Korea for the first time since 1953, in a symbolic gesture of reconciliation.

Passenger and freight trains crossed the heavily fortified border between North and South Korea for the first time since the Korean War divided the peninsula in 1953, a symbolically significant, if largely ceremonial and short-lived, gesture toward reconciliation between the two long-estranged nations during a period of improved inter-Korean relations.