[jameshart]

I think one of the most remarkable things about calendars is the endurance of ‘weeks’. For thousands of years, across civilizations, we’ve been counting off cycles of seven days, presumably successfully keeping in synch with one another, leading to the broad agreement across billions of humans that today is a Sunday.

[yen223]

This is true. Especially considering a "week" isn't tied to any specific astronomical phenomenon, unlike the day, month or year.

Maybe it succeeded because it's not tied to an astronomical phenomenon, and so we aren't tempted to botch it with "leap days" and whatnot.

[jameshart]

It is a lunar phase-cycle, roughly. Lunar phases last between 6 and 8 days, with a full 4-phase cycle taking about 29.5 days