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.
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.