Why the week has seven days
A day is one turn of the Earth, a month roughly one cycle of the Moon, a year one orbit of the Sun. The week is the odd one out. Seven days fit none of these cycles exactly, and nobody can point to the moment it was invented.
There are several likely roots, and they overlap. One is the Moon. A lunar cycle lasts about 29.5 days, so a quarter of it is a little under seven and a half. The Babylonians marked the 7th, 14th, 21st and 28th days of their lunar months as special. But that count started again with each month, so it was not yet a running week.
Another is the sky. Seven bodies visible to the naked eye move against the stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Seven was a significant number in ancient Mesopotamia.
The unbroken cycle of seven days, counting on regardless of the Moon, is first clearly seen in Judaism, tied to the Sabbath and the creation story in Genesis. It was in place by the sixth century BC at the latest.
Planets and Romans
The names came from astrology. By the first century AD, a seven-day week named after the Sun, the Moon and the five visible planets was widely known in the Roman Empire. The order of the days comes from a scheme in which each hour is ruled by a planet in turn, from the slowest, Saturn, to the fastest, the Moon. Because 24 hours leave three over when divided by seven, each day starts three planets further along: Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus.
English kept Saturday, Sunday and Monday, and swapped the rest for Germanic gods: Tiw, Woden, Thor and Frigg.
Rome had earlier used an eight-day market cycle. In 321, the Emperor Constantine decreed a seven-day week, with Sunday as a public holiday, and from there it spread across Europe and then the world.
Attempts to replace it did not last. Revolutionary France used ten-day weeks from 1793 to 1802. Between 1929 and 1940, the Soviet Union put most factory workers on five-day and then six-day working weeks. Both went back to seven.
So the week is not written in the sky. It is an agreement more than 2,500 years old that has proved very hard to change.
Where is an unbroken seven-day cycle, independent of the Moon, first clearly seen?
The seven-day week matches no cycle in the sky exactly. It is an inheritance from ancient traditions that has outlasted every attempt to replace it.
