Why are there seven days in a week?

Nighttime sky.

Ancient astronomers' observations of the night sky led eventually to our use of the seven-day week. Computer Science Geek photo. CC-BY.

Nobody knows for certain, but chances are that our seven-day week is historically connected with the fact that the Babylonians and other early astronomers recognized seven celestial bodies that weren’t in fixed positions like the stars — the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. In fact, in English the names of three of the days — Sunday (sun day), Monday (moon day) and Saturday (Saturn day) still are obviously related to those celestial objects.

As best we know, the seven-day week was first formally instituted by King Sargon I of Akkad (born in 2335 B.C.), who established the first Semitic dynasty in Mesopotamia. We have records of detailed astronomical observations being made by the Babylonians hundreds of years before then, as well as various festivals celebrated in honors of the gods of various planets, so it wouldn’t be surprising that the number seven would be chosen. It was also a convenient number, coming close to being able to divide a month (the period from one new moon to the next) to four roughly equal time periods.

Our adoption of the seven-day week also is directly tied to the Jewish use of the seven-day week, which was adopted by Christians (as Christianity began as an offshoot of Judaism) and eventually Constantine and the Roman empire. A seven-day calendar had been used by Romans at times before, but it was Constantine whose adoption of the calendar was most broadly accepted.

The interplay between the Babylonian calendar and the Jewish calendar is unclear. Some scholars believe that the Jews adopted the seven-day week when they were in Babylonian captivity beginning in the sixth century B.C. However, it may be that the Judeo-Christian creation account, which has God creating the universe in six days and resting on the seventh, was around long before the captivity (although it wasn’t until then that the creation account and the other portions of Genesis and other “books of Moses” were written in their final form). There is still much to be learned about how the Babylonians and the ancient Hebrews may have influenced each other prior to the captivity.

In any case, the Judeo-Christian seven-day week of the creation account, once adopted by Constantine, remained the standard of Europe long after Constantine was gone. Eventually, the dominance of Europe in world affairs led to the adoption of its calendar throughout the world.

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