Why Do Weeks Have Seven Days?

Sometimes, time divisions just make sense. For example, a day is intuitively defined as the duration of time from when your snooze button refuses to work until the nightly Chicago police/fire/medical/cosmetology disaster TV show wraps up another episode. Of course, some people consider a “day” as the amount of time it takes for the Earth to rotate once around its axis. Whatever.

There are other sensible divisions of time. The Earth orbits the sun once a year—every 365 days. Well, to get picky and offer up another trivia tidbit, it takes 365.25 days. That quarter day adds up, and if you do the simple math, it becomes apparent why there’s an extra day in leap years every fourth year.

So, there’s rhyme and reason to the year concept, too.


No, that’s not a typo. Months also have some logic backing their existence.

There are 12 lunar cycles during the Earth’s 365-day trip around the sun. Well, technically, a lunar cycle of all the phases is 29.5 days. Just in case you’ve heard a figure of 27 days, 7 hours, and 43 minutes, that’s how long it takes for the moon to orbit the earth. In either case, we’re in the reasonable ballpark of a “monthly” correlation. We just have to forgive the rounding errors—astrophysics is complex stuff, just ask Carl Sagan. Somewhere along the line, someone forgot to carry the one.

As the legend says, even the term “month” derives its name from the moon. If we lived in a true democracy, I’d vote they ought to be called “moonths,” but the powers to be have more pressing things to worry about, like making shady land deals.

What a Difference a Day Makes

So what about weeks? Why is a week seven days long? Why is this division roughly a quarter of a month or a fifty-second of a year? If you compare a week to a true lunar month, it works out to 23.728 percent of that. It makes no sense, offends my sensibilities, and the metric system is likely apoplectic over the sheer disorganization of the whole weekly concept.

Born in Babylonia… Moved to Arizona

While it wasn’t King Tut (Sorry, Steve Martin, it would have made a great lyrical addition to your song), it was King Sargon I of Akkad. He is ancient and most likely dead by now, having ruled around 2,300 B.C.

In that time and place, the number 7 held special significance. It’s also the number of major celestial bodies that can easily be seen by the naked eye: the sun, the moon, Jupiter, Mercury, Saturn, Mars and Venus. Remember, the Hubble Telescope didn’t go into service until April 25, 1990, one day after its launch. Well, its performance was limited at first due to a glitch in the mirror, but that’s a story for another day.

The Jews also favored a seven-day week. There’s the book of Genesis in the Bible, which outlines the concept of a seven-day time scale for creation including the seventh day of rest.

Of course, weeks are kind of arbitrary, and one can ask why we even have them at all. But people were enamored with the idea, even though, in absence of a celestial calibration orbiting bodies, they created “weeks” in their own likeness anyway. The Romans ran with an eight-day week. The Egyptians favored a 10-day week. Hard workers, those Egyptians.

So why did the magic number seven win out? The Jews were held captive by the Babylonians during the peak of Babylonian influence, and both cultures favored the concept of seven days for different spiritual reasons. The Persians and Greeks followed, then Alexander the Great, and we all know how much influence he had.

Eventually, even Rome shaved a day, and now we all live a septenary life.

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