Labor Day is the first Monday of September in the United States. It celebrates the labor movement, labor unions, an eight hour work day, etc. It’s often also considered the unofficial end to summer.
When I was a kid, it meant the last day before we started school again. That’s still true in some areas of the country, but now, some places have been doing school for a month or more.
I recommend you hop over to the Department of Labor’s website and read up more on the history of the holiday.