Black Friday is the day following Thanksgiving Day in the United States. It is often regarded as the beginning of the Christmas shopping season.
Most major retailers open very early (some at midnight) and offer promotional sales. Black Friday is not a federal holiday, but California and some other states observe "The Day After Thanksgiving" as a holiday for state government employees. Also many non-retail employees and schools have both Thanksgiving and the day after off, followed by a weekend, thereby increasing the number of potential shoppers.