The Dow Jones Industrial Average suffered its famous 12.8% loss on October 28, 1929, during the Wall Street Crash.
The day became known as Black Monday. Heavy selling overwhelmed the New York Stock Exchange, and the Dow fell 38.33 points, then a very large move relative to its level. The decline followed a sharp drop on October 24, later called Black Thursday, and came immediately before the even more famous October 29 collapse, known as Black Tuesday.
The crash did not happen in isolation. During the 1920s, many investors bought shares with borrowed money, while stock prices rose far faster than some underlying business conditions justified. The Federal Reserve had tightened monetary policy, and economic weaknesses were becoming more visible by autumn 1929.
A common mix-up is to identify October 29 as the first major crash day. October 29 was the most iconic session, but October 28 produced the larger percentage loss. The crash contributed to the wider economic catastrophe known as the Great Depression, although historians do not treat it as its sole cause.