The 1929 U.S. stock-market crash day that followed Black Thursday was known as Black Monday.
On October 28, 1929, the Dow Jones Industrial Average fell 12.8 percent. The decline came after the heavy selling of October 24, called Black Thursday, and before the even more dramatic fall on October 29, called Black Tuesday. Together, the sessions destroyed confidence in the overheated market.
The crash did not by itself cause every feature of the Great Depression, but it was a major symbol and accelerator of the economic collapse. Banks, companies, and households had become exposed to falling share prices, while margin borrowing magnified losses for investors who had bought stock with borrowed money.
Black Monday is sometimes confused with the 1987 crash, which is also widely called Black Monday. The 1929 label specifically refers to October 28, while the 1987 label refers to October 19, so the year is essential when identifying the event.