Black Monday began with a 12.8% fall in the Dow Jones Industrial Average on October 28, 1929.
On that Monday, the Dow dropped 38.33 points, or about 12.8%, as selling intensified after a sharp decline the previous Thursday. The following day, October 29, became known as Black Tuesday and brought another huge fall amid exceptionally heavy trading. Together, these sessions made the Wall Street Crash of 1929 a defining symbol of financial panic.
The crash followed a speculative boom in U.S. shares during the 1920s. Investors increasingly bought stocks on margin, meaning they used borrowed money, so falling prices generated margin calls and forced additional selling. The crash did not by itself cause every feature of the Great Depression, but it helped undermine confidence and was followed by wider banking and economic problems.
“Black Monday” is also the standard name for the global stock-market crash of October 19, 1987. The date is therefore essential: this question concerns October 28, 1929, not the later 1987 event.