In 1908, the first five-day workweek in the United States was instituted by a New England cotton mill so that Jewish workers would not have to work on the Sabbath from sundown Friday to sundown Saturday.
A 5 day work week is, like most things nowadays, arbitrary.
In the spirit of questioning assumptions, I'd like to eventually structure my life around a 4 day work week. Here's why:
Multiple experiments have been done around 4 day work weeks, and there has been no observable decrease in output. I believe a number things need to be in place for this, but it's certainly achievable.
People don't do much on Fridays. Unless you are a maker, in which case Friday afternoon can be the most productive time of the week, the last day of the week is usually lost around lunch time. Just check traffic stats on Friday afternoon on any website.
A longer weekend will give people enough time to unwind, take care of themselves, explore new interets and maximize social time. rest of the weekend shouldn't be used for Netflix. This should be time for reading, seeing friends, spending time doing some mobility work or physical activity.
The last reason, by itself, should outperform any short term productivity loss if done over a long enough period of time.