A week has seven days, a pattern inherited from ancient Babylonian and Jewish traditions.

The longer version

Seven is not an astronomical number; nothing in the sky divides time neatly into that span. Babylonian observers attached special weight to it, and the scriptural account of a creation completed and then rested on the seventh day fixed a rhythm of six working days followed by one of rest.

Because the cycle attaches to no celestial event, it runs continuously and never needs correcting, which helps explain its survival. Regimes that experimented with ten-day weeks usually reverted within a generation. The rhythm now shapes school timetables, shipping schedules and the pattern of rest days in most of the world. Its independence from the heavens makes it the one calendar unit a reformer cannot recalculate.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.