The event that marked the start of the Great Depression in the United States was the stock market crash of October 29, 1929, commonly known as Black Tuesday. On this day, panicked investors sold off millions of shares, leading to a monumental collapse in stock prices and a loss of confidence in the economy. This crash triggered widespread bank failures, high unemployment rates, and severe economic hardship for millions of Americans. The Great Depression had far-reaching effects on society, resulting in significant changes in economic policies and the way the government intervened in the economy.