the U.S. Department of Labor's website says, "Labor Day, the first Monday in September, is a creation of the labor movement and is dedicated to the social and economic achievements of American workers. It constitutes a yearly national tribute to the contributions workers have made to the strength, prosperity, and well-being of our country." That means unions, whether some people want to ignore how important they are.
Some people wrongfully try to demonize unions (just look at Fox News today, or better yet, don't bother), but this is foolish. Labor Day and Unions are more closely linked than peanut butter and jelly. Unions have done much for us, make sure you thank them on Labor Day.