The day after America's Thanksgiving holiday has become known as Black Friday, one of the biggest shopping days of the year.

It is the day when U.S. retailers traditionally turn a profit as millions of Americans rush out to stores in search of holiday gifts. The day places retailers in the black, in accounting terms.

Consumer spending accounts for about 70 percent of the U.S. economy and some retailers say end-of-the-year holiday spending accounts for about a quarter of their annual sales. But sales this year could increase just 2.5 percent compared to 2011, according to one estimate.

Holiday sales could be hurt by the sluggish pace of the U.S. economy, with more than 12 milllion workers still unemployed in the aftermath of the recession in 2008 and 2009.

Grey Thursday, a trend that began last year, has some chain stores offering a Thursday night start to the traditional post-Thanksgiving holiday shopping season.

Retailers are traditionally closed on Thanksgiving, known for its family gatherings with sumptuous turkey feasts.

Employees of some chain stores, however, have circulated several petitions online pleading with the public to “save Thanksgiving” to stop the Thursday openings so the employees can enjoy their Thanksgiving holiday.