The largest one-day percentage fall in the Dow Jones Industrial Average before 1987, recorded on October 28, 1929, was 12.8%.
October 28 became known as Black Monday during the Wall Street Crash of 1929. The Dow fell 38.33 points, a decline of approximately 12.8%, as panic selling intensified after weeks of market instability.
The following day, October 29, brought another historic decline, with the Dow falling about 11.7%. These sessions are often grouped together under the Wall Street Crash, although the crash continued into November and the market did not reach its ultimate low until 1932.
The 1929 collapse followed years of speculative buying, widespread use of margin, and an economy vulnerable to contraction. It helped deepen the Great Depression, but the crash itself was not the only cause of that worldwide economic downturn.