[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