Black Monday was the 1929 Wall Street crash day on which the Dow Jones Industrial Average fell nearly 13%.
The sharp decline occurred on October 28, 1929, after heavy selling had already shaken the market. The Dow dropped 38.33 points, a fall of about 12.82%, then lost another 30.57 points on Black Tuesday, October 29.
The crash did not by itself cause the entire Great Depression, but it damaged confidence, reduced wealth, and intensified financial stress. Other causes included bank failures, weak demand, excessive speculation, and international economic problems.
A common mix-up is the terminology. Black Thursday refers to October 24, the first major panic day in the famous sequence. Black Monday and Black Tuesday refer to the much steeper losses that followed. The Dow eventually lost almost 89% from its 1929 peak to its 1932 low.