Black Monday was the 1929 crash day when the Dow Jones Industrial Average fell 12.8%, its largest one-day percentage decline in that crash.
The date was October 28, 1929. Panic selling intensified after the market had already dropped on Black Thursday, October 24. On the following day, Black Tuesday, the Dow declined another 11.7%.
These labels describe the sequence of trading days rather than separate crashes. Black Monday and Black Tuesday are sometimes mixed up because Black Tuesday is more commonly associated with the public memory of the 1929 disaster.
The market’s decline continued after October 1929. The Dow did not reach its ultimate low until July 8, 1932, when it stood about 89% below its September 1929 peak.