The largest one-day percentage fall in Wall Street history, recorded on 19 October 1987, was Black Monday.
The Dow Jones Industrial Average fell 508 points, or 22.6%, in a single trading session. Markets dropped sharply across the world, making the event an international stock-market crash rather than only a United States episode. Hong Kong, London, Australia, and Canada were among the markets affected.
Several factors contributed, including rising interest rates, concerns about the dollar, trade tensions, and widespread use of computerized portfolio-insurance strategies. Those automated strategies could intensify selling when prices declined. The crash did not develop into a depression like the 1929 collapse, partly because central banks and financial authorities responded quickly.
A common mix-up is confusing Black Monday with Black Monday in 1929, a separate day during the Wall Street Crash. The 1987 event remains notable because the percentage decline was much greater than the one-day falls seen in 1929.