Black Friday is not an official holiday in the United States, but California and some other states observe "The Day After Thanksgiving" as a holiday for state government employees. It is sometimes observed in lieu of another federal holiday, such as Columbus Day. Many non-retail employees and schools have both Thanksgiving and the following Friday off. Along with the following regular weekend, this makes Black Friday weekend a four-day weekend, which is said to increase the number of potential shoppers.

Source: Wikipedia