Background image by G.Hüdepohl (atacamaphoto.com)/ESO via Wikimedia Commons.
There is more than one answer to this question. Nearly all of us use a seven day week but we do not all use the same seven day week. The names of the days of the week mean different things in different languages.
Let’s start with English. Sunday is clearly the day of the Sun. Monday sounds like it could be the day of the Moon. The only other day whose name gives anything away is Saturday, the day of Saturn. The other days are named after the gods Týr, Odin, Thor, and the goddess Frigg from Norse mythology.
You may not be familiar with these, except for Thor, but you will certainly be familiar with the Roman deities they were identified with during the early Christian era. Týr was identified with Mars, Odin with Mercury, Thor with Jupiter, and Frigg with Venus.
Most of us think of Mars, Mercury, Jupiter, Venus, and Saturn not as Roman gods but as planets. But why were planets worshiped as gods, alongside the Sun and the Moon, and given a day each? How did ancient peoples even know that these five planets are different from the thousand or so stars they could see in the night sky? Sure, Venus and Jupiter can be brighter than any star (other than the Sun of course) but that is certainly not true of Mercury. Is it that stars twinkle but planets do not? Yes but there is a more significant difference.
The word planet means “wanderer.” How exactly do planets wander? On a given night, they rise and set just like stars. But if you keep watching a planet month after month, you’ll see that its position doesn’t stay fixed with respect to the stars. Planets move from constellation to constellation. They wander relative to the stars. So do the Sun and the Moon.
In the majority of the world’s widely spoken languages, the days of the week are named after the Sun, Moon, and five visible planets.
But this is not the only seven day week. In many languages, the days are not named but simply numbered following the Judeo-Christian account of creation and the commandment to work for six days and rest on the seventh. In most Romance languages, the days from Monday to Friday are named after the Moon and four planets but Saturday refers to the sabbath and Sunday is “Lord’s Day” in commemoration of the resurrection of Christ.
Our seven day week has its origins in two distinct seven day weeks – a polytheistic one based on seven planet-gods and a Judeo-Christian one based on the biblical story of creation. Both of these merged into one in Rome during the first millennium CE.
Things need not have ended up this way. Seven day weeks were, by no means, universal in the ancient world. The Romans themselves had previously used an eight day week. The Egyptians used a ten day week. Six day weeks were not uncommon. Neither was not having anything resembling a week at all.
The planet-god seven day week might go back to the Alexandrian Greeks or the Neo-Babylonians. Most historians believe that the Babylonians first identified planets with gods and then the Greeks assigned days of the week to them. I have heard several explanations for the order of the days of the week but I don’t find any of them convincing enough to repeat here (we can discuss them in the comments section if you want).
I don’t know what inspired the seven day creation story in the Book of Genesis but the number seven probably wasn’t chosen because of the seven wanderers. Seven day periods are mentioned in Mesopotamia as far back as the 3rd millennium BCE, including in the Epic of Gilgamesh and in the Gudea cylinders, centuries before the earliest known mention of all seven wanderers.
The ancient fascination with the number seven is a mystery. There is no natural cycle which is seven days long. If it wasn’t the seven wanderers that first caught our attention, perhaps it was the seven bright stars of the Big Dipper or the Pleiades. Or perhaps it had nothing to do with astronomy. No, that couldn’t be it.
Sevens in the heavens. The image on the left, by GH5046 via Wikimedia Commons, shows the Big Dipper. The image on the right, by Takahashi Hososhima via Wikimedia Commons, shows the Pleiades star cluster next to the tree branches at the top-right.
The seven day week has endured various challenges during the last few centuries. A ten day week was used in France from 1793 to 1802 and for 18 days in 1871. Between 1929 and 1940, the USSR experimented with a five day week and then a six day week. Meanwhile, astronomers keep discovering additional wanderers.
The seventh planet, Uranus, was discovered in 1781. Ceres, discovered in 1801, was considered a planet for around 50 years until it was found to be part of a large collection of objects known as the asteroid belt. But just as Ceres lost its planet status, Neptune was discovered and so the number of planets remained at eight. Pluto was discovered in 1930 and was considered a planet until 2006. Its life as a planet would mirror that of Ceres, except with more drama. Again, the discovery of a large belt of objects, this time the Kuiper Belt, would strip an object of its planet status. Now, ten years later, some astronomers think there is evidence for a “real” ninth planet.
Imagine if the number of days in a week changed every few decades along with the official planet count.