In 1884 trade unions were demanding that work days be reduced from the typical 10-12 hours (6 days a week) down to a maximum of 8 hours. They set a deadline of May 1, 1886. When that deadline wasn't met, they held a peaceful protest in Chicago. On May 3rd, angry striking workers pushed toward some gates to confront strikebreakers / scabs. The police fired on the strikers, killing 6. The next day, there was a rally at Haymarket Square. At night, the police came in force to try to disperse the crowd. Someone threw a bomb at the police, killing one of them and severely wounding others. The police fired on the crowd, and some protesters fired back. At least 4 people were killed and at least 70 injured.
The result of all this, including the unfair trials, executions, pardons, etc. was a lot of attention to the 8-hour workday movement.
In 1890, the unions planned for another strike with the goal being the 8-hour work day. This time, with the help of the second Communist International, it went worldwide. The riot in Haymarket Square in Chicago on May 1 became a rallying cry for workers worldwide, and ever since then that has been the International Workers Day. But, in the US, the fact it was associated with communism was too scary, so the US celebration of Labour was moved to Sept 1st. Instead of International Workers Day, on May 1 the US celebrates (I kid you not) "Loyalty Day" and "Law Day" -- extremely rich given that the thing that kicked it off was a time when there was a bloody confrontation between cops and labour.
https://en.wikipedia.org/wiki/Haymarket_affair
A couple of decades later in the 1910s, as unions continued to push for an 8-hour work day, Henry Ford went with the 8-hour day in his factories, and that was so influential that it eventually became the norm.
The 5 day work week came after the 8 hour day. It was partially the result of Henry Ford deciding that it was more beneficial to give his workers 2 days off. It was also influenced by a cotton mill employing both Jewish and Christian workers arranging work schedules so each group could have its sabbath off. Once Ford made that rule, unions pushed extremely hard to make it a standard thing, but again, it took decades. It wasn't until 1940 that the Fair Labor Standards Act in the US made a 40 hour work week mandatory.
https://en.wikipedia.org/wiki/Workweek_and_weekend
The point of all this?
The 40-hour work week was never "designed". People fought and died to make it a reality.
People, mostly in unions, frequently communists, fought and died to gradually reduce the number of hours that workers were expected to work. In the mid 1800s the expectation was 6 days a week, 10-12 hours a day. It took decades of fighting to get that down to 6 days of only 8 hours. It took decades more fighting to get it down to 5 days a week rather than 6 or 5.5. It was never something that was "designed". It was something that took decades of battle.
White families in the US after WWII were the first to really benefit from a law which had gone into place just before the US entered the war. Those families benefited from decades of work from labour unions and communists to get the work week down to only 40 hours. Then, the economic boom the US received from being the only major country to come out of WWII with its infrastructure essentially untouched meant that for the first time, maybe ever, working-class families were living relatively comfortable lives. The man in the family went to work for the legal maximum 40 hours, and still earned enough to support a whole family without his wife needing to work outside the home.
What has happened since then isn't that the "designed" system failed. It's that the post-war economic boom ended as other countries recovered. It's that the labour unions got weak, and the capitalists started squeezing again. The 40-hour work week is still theoretically the law of the land. It's just that take-home pay has been stalled for decades as the cost of living has gone up.
Don't get me wrong, workers today still live better than the workers did in the mid 1800s when a work week was something like 60-80 hours. But, because labour unions got weak, and communism was demonized, there was nobody to oppose the owners of capital as they found new ways to squeeze their employees. So, even with a 40 hour week, things have been getting worse.
The history of the 40 hour week is also important because it shows what's going to be needed if people want to work less than 40 hours. People are going to need strong unions. They're going to need to go on strike. They're going to need to get hurt and maybe killed by the cops who will side with the bosses. And, once enough blood has been spilled, maybe there will be reforms. Complaining about it on social media and thinking that we just need to "design" a new mutually beneficial arrangement is missing the whole point.