Why are there seven days in a week?

Nothing in nature lasts seven days. A year is the Earth around the Sun; a month is roughly the Moon’s cycle; a day is one spin. The week is pure human invention.

The ancient Babylonians, 4,000 years ago, watched seven moving lights in the sky: the Sun, the Moon, and the five planets they could see — Mars, Mercury, Jupiter, Venus and Saturn. They gave each one a day, and the seven-day week was born.

You can still see those planets in the names. Sunday is the Sun’s day, Monday the Moon’s, Saturday Saturn’s. In Hindi and Malayalam the match is even clearer: Ravivaram (Sun), Somavaram (Moon), Mangalavaram (Mars), Budhavaram (Mercury), Guruvaram (Jupiter), Shukravaram (Venus), Shanivaram (Saturn).

The French tried a ten-day week after their revolution. It lasted twelve years. Nobody liked having one day off in ten.