7 days feels like it aligns well with the way we think and communicate about time. But maybe that is because it’s the only week length I’ve ever known.
One could imagine early humans being much more aware of the phase of the moon (whereas I haven't even seen that thing in months), and the 29.5 day cycle naturally splits into two pieces, since it's (new moon) -> (full moon) -> (new moon).
And one could also imagine people were pretty aware of when they were halfway between a new moon and a full moon — so now we've naturally arrived at a rough time interval of 29.5/4 = ~7 days.
Only 0.5 days out instead of 1.5.
Edit: though with the sinusiodal variation in the moon, I guess it makes sense to divide it into 4. 3 weeks doesn't make sense for that.
The French tried it for several years: https://en.wikipedia.org/wiki/French_Republican_calendar
The advantage being that you can do a lot more on your weekends (international travel on a weekend is now possible), and during the week you'd have a lot of uninterrupted time to work. I imagine the productivity would be similar to a five-day week but it would feel like a lot more time off.
We should go in the opposite direction - something like Weekend Wednesday [1] appeals to me. That way, you always have a break to look forward to, and you don't feel like your off days are unproductive. For longer breaks and international travel, that's what vacations and holidays could be for.
I could see that a one-day weekend might not seem long enough for some people, in which case I have a compromise: the current system. It isn't perfect for everyone, and isn't immune from change, but it's a decent enough middle ground.
[1] CGP Grey - https://youtu.be/ALaTm6VzTBw
https://yalebooks.yale.edu/book/9780300271157/the-week/
tldr: here's an interview he did on a 50 minute radio show about It (starts about 8.5 minutes in. I don't know why they distribute it this way either)
https://kpfa.org/episode/letters-and-politics-december-30-20...