Why Are There Seven Days in a Week? Babylon, the Planets and a Cycle Nobody Owns
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The day is the Earth turning, the month is roughly the Moon's cycle and the year is the Earth's orbit, but nothing in the sky takes seven days. The week is a human invention that has run without a break for more than two thousand years, across every calendar reform and revolution, and its days carry the names of Roman planets in half of Europe and of Norse gods in the other half. Where it came from is reasonably clear; why it proved impossible to abolish is more interesting.
Seven from Babylon
The Babylonians divided the lunar month of about 29 and a half days into quarters marked by the Moon's phases, which gives seven-day periods with an awkward day or two left over at the end of each month, and they attached special significance to the seventh, fourteenth, twenty-first and twenty-eighth days. Seven was already a sacred number to them, because there were seven visible moving bodies in the sky: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. The Jews, in exile in Babylon in the sixth century BC, took the seven-day cycle and made it continuous, cutting it free from the Moon so that it ran on regardless of the month, and anchored it to the Sabbath, the day of rest that the book of Genesis grounds in God's rest after six days of creation. A continuous week that ignored the sky was the innovation, and it is the one that survived.
The planetary week
The Romans used an eight-day market cycle until the seven-day week, spreading from the eastern provinces and from astrology, displaced it in the first centuries AD; Constantine made it official in 321 by declaring Sunday a day of rest. The astrologers had given each hour of the day to a planet in turn, in the order of their supposed distance from Earth, and the planet ruling the first hour of each day named the day. The names have come down in two streams:
- •Romance languages kept the Roman planets: lunes, martes, miercoles, jueves, viernes for Moon, Mars, Mercury, Jupiter and Venus in Spanish, with Saturday and Sunday replaced by the Sabbath and the Lord's day
- •Germanic languages translated the gods: Tuesday, Wednesday, Thursday and Friday are Tiw, Woden, Thor and Frigg, the Norse equivalents of Mars, Mercury, Jupiter and Venus, while Saturday kept Saturn and Sunday and Monday kept the Sun and Moon
- •Slavic and some other languages simply number them, so that Tuesday is the second day and Thursday the fourth
- •Chinese and Japanese adopted the cycle later and Japanese names the days for the Sun, Moon and the five elements, fire, water, wood, metal and earth, which are also the planets
The cycle that never broke
The week has run continuously since at least the time of Christ, and probably since the Babylonian exile, without a single day skipped or added. When Pope Gregory cut ten days from October 1582, Thursday 4 October was followed by Friday 15 October, and the week was preserved while the dates were not; the same happened when Britain lost eleven days in 1752. Two attempts to replace it failed instructively. Revolutionary France in 1793 introduced a ten-day week, with one day of rest in ten, and it was abandoned in 1802 after the population ignored it and the workers objected to the loss of rest days. The Soviet Union in 1929 introduced a five-day and then a six-day continuous week, with workers on staggered rest days so that factories never closed, and gave it up in 1940 after families found they never had a day off together. The cycle survives because it is a social rhythm, not an astronomical one, and everyone has to be on the same one.
Which day is first
The Jewish and Christian traditions count from Sunday, the day after the Sabbath, and the United States, Canada, Japan and most of the Middle East still print calendars that way. Most of Europe and the international standard for dates, ISO 8601, begin the week on Monday, treating Saturday and Sunday as the weekend that ends it. The weekend itself is recent: the two-day break arrived in Britain and America in the early twentieth century, when factory owners found that a Saturday half-day cut absenteeism on Monday, and Henry Ford adopted the five-day week in 1926. Muslim countries mostly rest on Friday, Israel on Saturday, and the world's business runs on a cycle that different faiths count from different days and that none of them can shift by an hour.
Working out the order of the days
The order of the weekdays looks arbitrary, but you can work it out yourself. Ancient astronomers ranked the seven moving bodies by how slowly they seemed to move against the stars, which they took as their distance from Earth: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. Astrologers gave the first hour of a day to one of them and each following hour to the next in that list, cycling round. A day has 24 hours, and 24 divided by 7 leaves 3, so the ruler of the first hour of the next day is always three places further along the list.
Start with Saturn's day. Count three places on and you reach the Sun, so the next day is Sunday. Three places after the Sun is the Moon, giving Monday. From the Moon you wrap round the list to Mars, giving Tuesday, the day Latin called dies Martis. Then comes Mercury, Wednesday; Jupiter, Thursday; Venus, Friday; and three places after Venus brings you back to Saturn and Saturday. The whole familiar sequence falls out of a list of seven planets and a 24-hour day, which is strong evidence that the names were fixed by this astrological scheme rather than chosen one by one.
Four-day, five-day and ten-day weeks
The seven-day week is not the only cycle people have run. Many societies used a market week, set by how often traders could rotate between neighbouring towns rather than by anything in the sky. The Igbo of what is now Nigeria count a four-day week, its days named Eke, Orie, Afo and Nkwo, and markets in different villages are still held on particular days of it. In Java a five-day market week, with days called Legi, Pahing, Pon, Wage and Kliwon, runs alongside the seven-day week, and the two together make a 35-day cycle that is still used to mark birthdays and choose good days for ceremonies.
Ancient Egypt's civil calendar divided each 30-day month into three ten-day periods, and the Maya counted a sacred cycle of 260 days by meshing 13 numbers with 20 named days. Combining cycles works like gears with different numbers of teeth: two cycles return to the same starting pair only after a number of days equal to the lowest common multiple of their lengths, 35 for five and seven, and 260 for 13 and 20. The week and the 365-day year mesh the same way: since 365 days is one more than 52 weeks, a given date moves on by one weekday each year, or by two after a leap day.
Languages that number their days
Not every language names its days after gods or planets. Portuguese numbers most of them: Monday is segunda-feira, the second day, Tuesday terca-feira, the third, and so on to Friday, the sixth, with Saturday and Sunday called sabado and domingo. The system comes from church usage in which Sunday counted as the first day of the week.
Many Slavic languages name some days by their position after Sunday too. In Polish, Wednesday is sroda, the middle, and Thursday czwartek, the fourth, while Friday is piatek, the fifth. Chinese also uses numbers, calling Monday week one and so on. Comparing languages shows that the seven-day cycle itself spread far more widely than any single set of names for its days.
The takeaway
The seven-day week comes from Babylon, where the lunar month was quartered and seven was the number of visible planets, and from the Jews, who made the cycle continuous and anchored it to the Sabbath; the Romans adopted it with days named for the planets, which the Germanic peoples translated into their own gods. Unlike the day, month and year it answers to nothing in the sky, it has run unbroken through every calendar reform, and the French and Soviet attempts to replace it collapsed because a week is something everyone has to share.