Black Monday was the day of the largest one-day percentage fall in the Dow Jones Industrial Average during the 1929 Wall Street Crash.
On October 28, 1929, the Dow fell 12.82%, after already suffering heavy losses on the preceding Thursday. Panic selling intensified as investors rushed to exit positions purchased with borrowed money. The following day, October 29, became known as Black Tuesday and brought another huge decline, but Black Monday was the larger percentage loss.
The crash did not by itself cause the entire Great Depression, although it severely damaged confidence and wealth. Bank failures, falling industrial production, deflation, reduced consumer spending, and policy mistakes also deepened the economic collapse. A common mix-up is treating Black Tuesday as the single worst day; Black Tuesday was more famous, but Black Monday had the greater percentage drop in the Dow.