In general, units of time can be divided into two categories. One category contains the units that measure something objective and observable, typically the movements of astrological objects. A day, for example, is the length of time it takes our planet to complete one rotation with respect to the sun. The second category is much more fun: totally random, basically meaningless divisions of time that were created out of a combination of superstition, incorrect science, and the need for greater precision in timing.
The seven-day week is in the latter category. There’s no good reason for it, and yet, it’s constant to almost every single culture.
Jews, who use a lunar calendar made up of either 12 or 13 months beginning with the New Moon, use a seven-day week. The Bengali calendar, which splits the year up into six seasons of two months each, uses a seven-day week. Even the Bahá’í, with their 19-month (and change) year, use a seven-day week.
Seven-day weeks very rarely divide evenly into any month or lunar division. They don’t fit into the overarching sexagesimal system; they don’t divide evenly into any conception of a year. Even our hours, minutes and seconds make more sense than the week. The sexagesimal system, meaning a numeral system with 60 at its base, happens to be fantastically flexible and information-dense
The week’s slender hold on linear time accounting hasn’t gone unnoticed. Throughout history, into the 20th century, thinkers have tried to oust the seven-day week for various philosophical, mathematical, and political reasons.
And yet, the damn thing persists.
We don’t really know where the 7-day week originated, but there are some existing theories about why a period of around that length would make sense. “Only by establishing a weekly cycle of an unvarying, standard length could society guarantee that the continuity of its life would never be interrupted by natural phenomena such as the lunar cycle,” writes Eviatar Zerubavel in his book, The Seven Day Circle: The History and Meaning of the Week. In other words, this is all the moon’s fault for being so unreliable.
Zerubavel especially links the need for an interval of this length to the rise of market culture: there needed to be an agreed-upon time in which vendors and buyers could meet, and about four times every lunar cycle seemed a pretty good frequency.
Alternate-number weeks used to be common. The Egyptians, for example, used 10-day weeks. But our best guess for the creation of the seven-day week is that the idea originated in ancient Babylonia. The Babylonians, living in Mesopotamia (present-day Iraq), held the number seven as a holy number, that being the number of objects in our Solar System they could observe at the time: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. The Babylonians already had months, just like anyone else, and if you want to split up a period of around 29 days into a smaller period, why not divide it basically into four parts, especially when that number is damnably close to your holy number of seven? So the Babylonians used a seven-day week, with the seventh day having certain religious responsibilities (relaxation, cessation of work, worship, that kind of thing). Writes Zerubavel:
The length of the astrological week was largely a result of the fact that the ancient Babylonian astronomers happened to identify seven planets. (Had they been able, with the help of some sophisticated telescopes, to observe Uranus, Neptune, and Pluto, the week might have evolved as a ten-day cycle. On the other hand, had they accepted our heliocentric model of the planetary system, where the sun is considered a stationary star rather than a planet, this cycle might have been only six days long.)
Around the 6th century BC, the Babylonians were a dominant culture in the Near East, and their ideas spread far and wide, including the concept of the seven-day week. The Jews happened to be captives in Babylonia around that time, and adopted the week concept. So did the nearby Persians and the (not yet dominant) Greeks. The Jews already agreed with the Babylonians that “7” is a very cool number indeed; creation in the Jewish tradition (and Jewish-derived traditions, like Christianity) took place in seven days. (Though it’s worth noting two things: first, nobody’s totally sure that the Jewish creation myth actually predates the Babylonian captivity, and second, that “days” in that case probably translates better to something like “periods” or “intervals.”) Anyway, the Jews got on board.
The Greeks did, too, and when they began traveling and conquering around the time of Alexander the Great, from 356 to 323 BC, they brought the seven-day week with them as far east as India and, either directly or, more likely, through Indian contacts, to China. The last major society to fall to the tyranny of the seven-day week was the Romans; they had observed a strict 8-day week up until 45 BC, when the Romans adopted the new Julian calendar, which is extremely similar to the Gregorian calendar we use today. The Julian calendar used seven-day weeks, but the Romans observed, weirdly, both seven-day Julian weeks and (to a smaller degree) the older eight-day cycles until Constantine officially banned the eight-day cycle in 321 AD. By that time the eight-day cycle was barely used.
Every once in awhile, somebody comes along and tries to change the seven-day week. Some scientists and sci-fi fans like to amuse themselves with metric time, in which the base unit of time (to replace the second, and thus the minute, hour, etc.) is something like a hundred-thousandth of a day. Thus a “metric minute” (which would be called something sci-fi and weird) would be a thousandth of a day, and a “metric hour” (also something weird) would be a tenth of a day, or something like that. Usually a week in these schemes would be 10 days long, because 10 is a nice number.
The most recent real attempt to throw out the seven-day week was in 1929, when the USSR changed the calendar to have 72 weeks of 5 days each (the difference was made up with some national holidays, which did not fall on normal days of the week—it’d be like having Saturday, Sunday, Labor Day, Monday, Tuesday). Each worker was given one of these days as a rest day, in which they didn’t work. The system was designed to make for a continuous work week; at any given moment, 24 hours a day, every day, 80 percent of the work force was working, compared with in garbage capitalist systems that gave universal weekends.
The system proved very frustrating; most people did not have the same rest day as their spouses, or friends, or family. And machines broke down for the same reason that New York City subways are always breaking down. When a system works 24 hours a day, every day, there’s no real time to repair or maintain them. In 1931, the USSR changed their schedule to…a six-day week, in which every sixth day was a rest day. This proved not much better, and in 1940 the USSR gave up and went back to a seven-day week.
The weekend, too, is a fairly recent creation. One day of rest per 7-day week is still, in many parts of the world, the standard, but in the US, that changed in the early decades of the 20th century. Factories with large populations of Jewish workers began, starting in 1908 with a mill in New England, to just give both Saturday and Sunday off. Henry Ford also implemented a two-day weekend. Soon labor unions began demanding it, and in 1938, FDR’s signing of the Fair Labor Standards Act established a five-day, 40-hour workweek for most American workers.
There’s nothing in particular about a 7-day week that makes it a requirement for anybody to observe; it seems that the idea took off simply because there was a need for a unit of time somewhere between five and 10 days long, and seven was a cool number. What’s surprising is that humans haven’t come up with anything better, except maybe, New Age app gurus.