The sharp U.S. stock-market fall on October 28, 1929, is commonly called Black Monday.
On that Monday, the Dow Jones Industrial Average fell about 13 percent. The decline followed heavy selling on Black Thursday, October 24, and came one day before the even larger fall remembered as Black Tuesday, October 29. Those three sessions became the most famous opening phase of the Wall Street Crash.
The crash did not by itself cause every feature of the Great Depression, but it destroyed wealth, damaged confidence, and intensified financial pressure. Speculation had pushed share prices far above sustainable levels, while many investors had bought stocks with borrowed money. Falling prices therefore produced forced selling and losses for brokers, banks, and households.
Black Monday is sometimes confused with Black Tuesday because the two dates were consecutive and both brought exceptional losses. The question’s date identifies the Monday event specifically.