The record-setting 19 October 1987 trading day was called Black Monday.
On that day, the Dow Jones Industrial Average fell 508 points, or 22.6%, its largest one-day percentage decline. The sell-off spread rapidly across international markets, including Hong Kong, London and Australia, making the event a global financial shock rather than an isolated U.S. decline.
Several factors were associated with the crash, including overvalued shares, trade and interest-rate concerns, portfolio insurance and computerized selling. Researchers have not identified one universally accepted single cause. The market’s rapid fall also exposed weaknesses in trading and settlement procedures.
The name can be confused with Black Monday events in other financial crises. In the stock-market context, “Black Monday” most commonly identifies 19 October 1987. The crash later encouraged reforms such as circuit breakers and coordinated market oversight.