How did a week come to have seven days?

The Moon returns to the same phase about every 29.5 days. Dividing the cycle into new moon, first quarter, full moon, and last quarter gives about seven and a half days per part. It isn't exactly seven.

Still, it was an easy number to count for people watching the Moon with their own eyes. Ancient Mesopotamian records show traces of treating the 7th, 14th, 21st, and 28th days of the lunar month as special. But that count started over with each new moon. The custom of seven days running on endlessly, regardless of the Moon, took hold separately.

In Roman times, a week that named each day after one of the bodies visible to the naked eye spread: the Sun, the Moon, Mars, Mercury, Jupiter, Venus, and Saturn. Overlapping with the Jewish tradition of a day of rest every seven days, the seven-day week took root across the Mediterranean.

Korean day names share the same roots. Wol is the Moon, il is the Sun, and hwa, su, mok, geum, and to are the five planets. In English, Sunday, Monday, and Saturday still carry the Sun, the Moon, and Saturn. These names reached East Asia long ago, and Koreans began using them in modern times along with a new calendar.

During the French Revolution, months were divided into ten-day periods. The arithmetic was neat, but days off dropped to once every ten days, and market days and days of rest were already tied to seven days, so it was abandoned after a little over a decade. The Soviet Union also tried five-day and six-day weeks before going back to seven.

The week keeps running without a break even when a month or year changes. That's why 365 days is 52 weeks plus one day, and your birthday falls one weekday later each year. If February 29 comes in between, it moves two days.

The seven-day week has lasted this long more because of daily life than the sky. Once market days, days off, and gathering days are set to seven days, no one can change it alone unless everyone changes together.

So the week is closer to an agreement among people than a unit set by calculation. If Monday feels heavy, it isn't Earth's fault. It's because that agreement has carried on for thousands of years.