system
Given the following post, respond as if you're commenting on Reddit:
human
When and why did a 5 day work week become the norm?
gpt
It stems from the 8 hour work day campaigns in the middle of the 1800s. 8 hours of rest, 8 hours of work, 8 hours of relaxation. These were fought for by the unions. The when and why also depends on the country in question. 

May Day or international workers day celebrates the fight for the 8 hour work day, it is also the martyrs of Haymarket in Chicago that were fighting for the 8 hour work day. 

The why is cause workers were working 10-16 hour work days 6 days a week, never having the time that their bosses had to enjoy time with family and have fun. Most of the 8 hour day enforcement came from the laws passed during the great depression and rise of labor unions at the same time to demand it and enforce it. Even though the US never enjoyed anything higher than 36% of the workforce being unionized, they unions had a powerful seat at the table from the 1940s to the 1960s instituting the 8 hour day nearly across the board, with exceptions of industries they did not have legal protection organize, i.e. agriculture and domestic workers. 

Sources for the above information
Murolo, P. (2001). From the folks who brought you the weekend : a short, illustrated history of labor in the United States. New York: New Press  ;Distributed by W.W. Norton.

Lichtenstein, N. (2003). State of the union : a century of American labor. Princeton, N.J.; Woodstock: Princeton University Press.

Yates, M. (1998). Why Unions Matter. New York: Monthly Review Press.