The global stock-market plunge of October 1987 is commonly called Black Monday.
The name refers especially to Monday, October 19, 1987, when markets around the world fell dramatically. The Dow Jones Industrial Average suffered its largest one-day percentage decline, while equity markets in places such as London, Hong Kong, and Australia also recorded major losses during the episode.
The label can cause confusion because “Black Monday” has been used for other financial events, including the 1929 U.S. market decline on October 28. In standard financial-history usage, however, Black Monday most often means the worldwide 1987 crash. Regulators later introduced coordinated safeguards, including trading halts and circuit breakers.