The 1987 market crash that began with a record one-day fall in Hong Kong’s Hang Seng Index was Black Monday.
On October 19, 1987, the Dow Jones Industrial Average fell 22.6%, its largest one-day percentage decline. The worldwide sell-off had already become severe in Hong Kong, where the Hang Seng Index dropped 46% between October 19 and October 26. Markets in Europe, Australia, Canada, and other regions also recorded major losses.
Several explanations have been studied, including overvalued markets, program trading, portfolio insurance, and international contagion. No single cause fully explains the speed and scale of the collapse.
This question is distinct from asking for the Dow’s largest percentage fall: the specific global event is conventionally named Black Monday, while Hong Kong’s market closed for several days during the turmoil and resumed trading amid continuing pressure.