Campaign by Social Reformer Robert Owen in 1810.
Why 5 day work weeks?
Out of the seven days, originally six days were for work and a single day was given off.
Religion / region-specific, ie. Fridays for Muslims, Saturdays for Jews and Sundays for Christians.
5-day work weeks, because in the US Saturdays were off for Jews (Sabbath) and Sunday for Christians.
Why 40 hour work weeks?
To keep the factories going 24/7 During the Industrial Revolution folks worked 100 hours (avg. 16 hour shifts).
September 3, 1916: The US Congress passes the Adamson Act to establish an 8-hour workday for interstate railroad workers.
September 25, 1926: Ford Motor Company becomes the first major company to adopt a 5-day, 40-hour workweek.
June 26, 1940: A provision in the 1938 the US Fair Labor Standards Act mandates a maximum 40-hour workweek with a 2-day weekend which is adopted nationwide.
With time most of the world adopts the US 5 day / 40 hour work week as the the standard due to US / EU led International markets.