During the 1929 Wall Street Crash, 28 October was called Black Monday, when the Dow fell 12.8%. The decline was the largest one-day percentage fall in the Dow Jones Industrial Average up to that time.
Black Monday followed Black Thursday, 24 October, when heavy selling first disrupted confidence. On 29 October, the market suffered another severe fall, a day remembered as Black Tuesday. Together, these sessions marked the most dramatic phase of the crash.
The crash did not cause the entire Great Depression by itself. Economic weaknesses included overproduction, falling farm incomes, bank failures, debt, and reduced consumer spending. The market collapse intensified these problems and damaged businesses, investors, and financial institutions.
A common mix-up is treating Black Monday as the single name for the whole 1929 crash. It specifically refers to 28 October 1929, while Black Tuesday refers to the following day.