Black Monday saw the largest single-day percentage fall in the Dow Jones Industrial Average during the 1929 Wall Street Crash.
On October 28, 1929, the Dow fell 12.82%, after already suffering heavy losses on the preceding Thursday. The next day, October 29, became known as Black Tuesday and brought another decline of about 11.7%. Although Black Tuesday is the more famous label, Black Monday produced the larger percentage drop in the index.
The crash followed years of speculative buying, widespread use of borrowed money, and exceptionally high share valuations. Banks and investors had become deeply exposed to falling prices, so the collapse damaged both financial institutions and households. The crash did not by itself create every condition of the Great Depression, but it intensified a broader economic downturn. A common mix-up is treating Black Thursday, Black Monday, and Black Tuesday as one date; they were separate sessions in a dramatic sequence of late-October 1929 trading.