Black Friday is an informal name for the Friday following Thanksgiving Day in the United States, which is celebrated on the fourth Thursday of November. Many stores offer highly promoted sales on Black Friday and open very early (sometimes as early as midnight), or may even start their sales at some time on Thanksgiving. Black Friday has routinely been the busiest shopping day of the year in the United States at least since 2005.The earliest evidence of the phrase Black Friday originated in Philadelphia, where it was used by police to describe the heavy pedestrian and vehicular traffic that would occur on the day after Thanksgiving. This usage dates to at least 1961. 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".There have been reports of violence occurring between shoppers on Black Friday. Since 2006, there have been 12 reported deaths and 117 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. Environmentalists cite one more adverse factor: discount deals encourage people to purchase things they don't need, and this overproduction contributes to climate change.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 attempted to promote the day to remain competitive with US-based online retailers.