In the United States, the Friday following Thanksgiving Day is known as Black Friday. On Black Friday, many retailers offer heavily advertised bargains and open early (often as early as midnight) or on Thanksgiving Day. The length of the shopping season is not the same across all years: the date for Black Friday varies between November 23 and 29. Since at least 2005, Black Friday has been the busiest shopping day of the year in the United States. For millennia, the term "black" has been used to describe days when disasters struck. The term "Black Friday" was first used to refer to the day after Thanksgiving. It was used to describe the practise of employees calling in ill the day following Thanksgiving to get a four-day weekend. In more recent decades global retailers have adopted the term and date to market their own holiday sales. Although "The Day after Thanksgiving" is not an official holiday in the United States, it is observed by state government employees in California and a few other states. Thanksgiving Day and the following Friday are both off for many non-retail jobs and schools. This makes Black Friday weekend a four-day weekend when combined with the next regular weekend, which is thought to boost the number of prospective shoppers.