"It is appropriate, therefore, that the nation pay tribute on Labor Day to the creator of so much of the nation's strength, freedom, and leadership — the American worker." US Dept of Labor
While Wikipedia has much to say about the American holiday, as well as international observances to recognize workers, many Americans see the first Monday in September as summer's last hurrah.
In some parts of the country, school starts in August or earlier. In Western Washington state, however, school typically starts the Tuesday or Wednesday following Labor Day.
This first 'last' Monday is spent on one last swim, the last big barbecue, last-minute buying school supplies, packing lunches, setting out 'first day of school' clothes, and trying to get to bed early enough to catch the bus in the morning.
Mom's going swimming.