October 19, 1987, is the trading day remembered as Black Monday in Hong Kong, Britain, and the United States.
The crash moved across time zones. Hong Kong markets fell first, European markets followed, and the United States then experienced its historic one-day Dow decline. Because October 19 was a Monday in New York, the date became the event’s most familiar name even though the international sell-off unfolded over several calendar days.
The Dow’s 22.6% fall was the largest one-day percentage decline in its history. Britain’s FTSE 100 also suffered a huge fall, and several other markets recorded losses of around 20% or more.
The episode demonstrated the increasing integration of global financial markets. It also prompted reforms to trading infrastructure, including coordinated circuit breakers and stronger mechanisms for handling extreme volatility.