What was the name of the 1929 crash day when the Dow lost nearly 12.8%?

The 1929 crash day when the Dow lost nearly 12.8% was called Black Monday.

Black Monday occurred on October 28, 1929, after heavy selling resumed on Wall Street. The Dow Jones Industrial Average fell 38.33 points, or about 12.8%, in one session. The following day, Black Tuesday, brought another huge decline and is the date most commonly associated with the crash.

The crash did not by itself cause every feature of the Great Depression, but it destroyed wealth, damaged confidence, and exposed weaknesses in the financial system. Margin buying had allowed investors to purchase shares with borrowed money, increasing both gains and losses. Black Monday is therefore distinct from Black Tuesday, even though both were part of the same 1929 collapse.