The Wall Street Crash of 1929 recorded its biggest one-day percentage decline on October 28, 1929.
That Monday, often called Black Monday, the Dow Jones Industrial Average fell 12.82 percent. The following day, Black Tuesday, brought another enormous decline of 11.73 percent and is frequently remembered more widely because selling intensified and trading volume reached a record for the period.
The crash followed years of speculative buying, widespread use of borrowed money, and rapid expansion of share prices during the 1920s. The collapse did not by itself cause every feature of the Great Depression, but it severely damaged confidence and helped deepen the economic crisis.
October 24, 1929, known as Black Thursday, was an earlier panic day, not the largest percentage fall. The market continued declining after October 29, reaching a much lower point in 1932.