Labor Day is a federal holiday that came about to recognize the labor movement and the power of collective bargaining. By the late 1800s, various trade unions were sponsoring days to recognize their workers, and it made sense to select one day as a "general holiday for the laboring classes."
The first Monday in September was chosen for the day, which became official in 1894. Many other countries were adopting May 1 – May Day – as a day to recognize laborers (later this would become International Workers' Day). President Grover Cleveland, among others, was concerned that adopting May 1 would have socialist and anarchist overtones because of the proximity to and association with the anniversary of the Haymarket Uprising in Chicago (May 4, 1886). The September date was much less radical.
Labor Day is often viewed as "the end of the summer" although summer does not officially end until September 21. Many schools and sports activities start around Labor Day, and many resort areas consider this the end of the tourist season. In fashion, traditionally it was the last day of the year that wearing white or seersucker was acceptable.
Here are some vintage postcards that celebrated Labor Day. Note the one that announces it as the latest holiday to be added to the calendar; there would still be more holidays to come.