One of the classic answers comes from British labor historian E.P. Thompson, who ties the creation of labor-time with industrialization. Workers often worked Tuesday through Saturday, then not Sunday followed by "St. Monday," a day of absenteeism. This tradition changed in the 19th century (instead of St. Monday there was more commonly a half day on Saturday). The more modern notion of the work week really arrives in the 20th century with the labor reforms birthed first in France under the Popular Front (the 40-hour work week).