October 1929 was the first month of the 1929 Wall Street Crash, when the Dow Jones Industrial Average began its historic collapse.
The market had reached a major peak in September 1929, but selling intensified in October. Black Thursday came on October 24, followed by Black Monday on October 28 and Black Tuesday on October 29. These sessions made the collapse internationally famous.
The crash continued beyond October, so describing it only as an October event can be misleading. The Dow kept falling into 1932, when it reached its low for the bear market. Another common confusion is to equate the crash with the whole Great Depression; the crash contributed to the depression, but many other financial and economic forces were involved.