Black Monday was the 1929 trading day associated with the largest one-day percentage fall in the Dow Jones Industrial Average at that time.
On October 28, 1929, the Dow dropped 12.82%, then fell another 11.73% on October 29, commonly called Black Tuesday. The two-day collapse followed weeks of heavy selling after years of speculative buying and margin trading. Black Thursday, October 24, had already shaken confidence, although financiers temporarily supported major stocks that day.
The crash did not single-handedly cause the Great Depression. It damaged wealth and confidence, but bank failures, falling demand, monetary policy, debt, and international economic problems also deepened the downturn. The Dow continued falling until July 1932, when it stood roughly 89% below its September 1929 peak.
A common mix-up is treating Black Monday and Black Tuesday as interchangeable. They were consecutive sessions, but Black Monday came first and had the larger percentage decline.