Labor Day is not just the popular end of summer. It is a day to reflect on the rights of workers. The Industrial Revolution brought crowds of people into the cities to work in factories for shamefully low wages. In the 16th century only a fifth of the population was poor. In the 19th century a third of the population was poor.
In the United States in the second half of the 19th century workers organized as the Knights of Labor. The president, Terence Powderly, and two-thirds of the organization were Catholic. In September, 1882, in New York City, they organized the first Labor Day parade. Cardinal Gibbons, the Archbishop of Baltimore, made a special trip to Rome in 1886 to convince the pope that the Knights of Labor was a worthy organization, deserving Church support.
In 1891 Pope Leo XIII issued the encyclical Rerum Novarum (literally new things) on the condition of labor. Among many "new things," he declared that workers had a right to a living wage and also that workers had the right to organize into unions. Other popes repeated and broadened Leo's support of organized labor: Pius XI, John XXIII, and Paul VI. It has become an essential part of Catholic moral theology.