Labor Day is a United States federal holiday observed on the first Monday in September, that celebrates the economic and social contributions of workers. According to Wikipedia, the date chosen was simply "convenient because it was midway between the Fourth of July and Thanksgiving". It was first nationally recognized in 1894 to placate unionists following the Pullman Strike. With the decline in union membership, the holiday is generally viewed as a time for barbeques and the end of summer vacations.
Labor Day has come to be celebrated by most Americans as the symbolic end of the summer. In high society, Labor Day is (or was) considered the last day of the year when it is fashionable to wear white or seersucker.
It's hard to say for sure, but an article in Time Magazine says that it had to do with the well-heeled wearing white during their summer vacations and then changing back to dark colors for when they returned to the sooty, dusty city.
To take advantage of large numbers of potential customers free to shop, Labor Day has become an important sale weekend for many retailers in the United States. Some retailers claim it is one of the largest sale dates of the year, second only to the Christmas season's Black Friday.
Ironically, because of the importance of the sale weekend, some of those who are employed in the retail sector not only work on Labor Day, but work longer hours.