Black Friday is an informal name for the day following Thanksgiving Day in the United States, the fourth Thursday of November, which has been regarded as the beginning of the country's Christmas shopping season since 1952, although the term "Black Friday" did not become widely recognised or used until the 1980s or later in some regions.

Most major retailers open very early, as early as overnight hours, and offer promotional sales. Black Friday is not an official holiday, but California and some other states observe "The Day After Thanksgiving" as a holiday for state government employees, sometimes in lieu of another federal holiday, such as Columbus Day. Many non-retail employees and schools have both Thanksgiving and the following Friday off, which, along with the following regular weekend, makes it a four-day weekend, thereby increasing the number of potential shoppers.

Black Friday has routinely been the busiest shopping day of the year in the United States since 2005, although news reports, which at that time were inaccurate, have described it as the busiest shopping day of the year for a much longer period of time. Similar stories resurface year upon year at this time, portraying hysteria and shortage of stock, creating a state of positive feedback.

In 2014, spending volume on Black Friday fell for the first time since the 2008 recession. $50.9 billion was spent during the 4-day Black Friday weekend, down 11% from the previous year. However, the U.S. economy was not in a recession. Christmas creep has been cited as a factor in the diminishing importance of Black Friday, as many retailers now spread out their promotions over the entire months of November and December rather than concentrate them on a single shopping day or weekend.

The earliest evidence of the phrase Black Friday applied to the day after Thanksgiving in a shopping context suggests that the term originated in Philadelphia, where it was used to describe the heavy and disruptive pedestrian and vehicle traffic that would occur on the day after Thanksgiving. This usage dates to at least 1961. More than twenty years later, as the phrase became more widespread, a popular explanation became that this day represented the point in the year when retailers begin to turn a profit, thus going from being "in the red" to being "in the black".

For many years, it was common for retailers to open at 6:00 a.m., but in the late 2000s many had crept to 5:00 or 4:00. This was taken to a new extreme in 2011, when several retailers (including Target, Kohl's, Macy's, Best Buy, and Bealls) opened at midnight for the first time. In 2012, Walmart and several other retailers announced that they would open most of their stores at 8:00 p.m. on Thanksgiving Day, prompting calls for a walkout among some workers. In 2014, stores such as JCPenney, Best Buy, and Radio Shack opened at 5:00 PM on Thanksgiving Day while stores such as Target, Walmart, Belk, and Sears opened at 6:00 PM on Thanksgiving Day. Three states, Rhode Island, Maine, and Massachusetts, prohibit large supermarkets, big box stores, and department stores from opening on Thanksgiving, due to what critics refer to as blue laws. The Massachusetts ban on forcing employees to work on major holidays is not a religion-driven "blue law" but part of the state's Common Day of Rest Law. A bill to allow stores to open on Thanksgiving Day was the subject of a public hearing on July 8, 2017.

There have been reports of violence occurring between shoppers on Black Friday. Since 2006, there have been 10 reported deaths and 111 injuries throughout the United States. It is common for prospective shoppers to camp out over the Thanksgiving holiday in an effort to secure a place in front of the line and thus a better chance at getting desired items. This poses a significant safety risk, such as the use of propane and generators in the most elaborate cases, and in general, the blocking of emergency access and fire lanes, causing at least one city to ban the practice. Since the start of the 21st century, there have been attempts by retailers with origins in the United States to introduce a retail "Black Friday" to other countries around the world. In several countries, local retailers have, maybe inspired by TV shows originating from the US, attempted to promote the day.