What was the name of the two-day 1929 sell-off when the Dow fell 23%?

The two-day 1929 sell-off when the Dow fell about 23% was called Black Monday and Black Tuesday.

On October 28, 1929, the Dow Jones Industrial Average dropped 12.8%, and on October 29 it fell another 11.7%. Together, the two sessions produced a decline of roughly 23% and represented the most severe phase of the initial panic. Newspapers and later historians used the names Black Monday and Black Tuesday to distinguish these dates.

The selling followed a sharp rise in U.S. share prices during the 1920s. Many investors bought stocks with borrowed money, so falling prices forced margin calls and additional selling. Investment trusts, banks, and individual investors were all affected as confidence collapsed.

Black Thursday, October 24, had begun the public panic, but it was not the largest two-day percentage fall. Black Tuesday is also sometimes confused with October 29, 1987, but the 1987 crash was a separate event. The 1929 crash continued beyond these sessions, with the Dow ultimately reaching a much lower bottom in 1932.