A unit with no astronomical basis
The day, the month and the year each have a physical cause. The week does not. A seven-day cycle matches nothing in the sky, although it roughly equals a quarter of the 29.5-day lunar month, and that rough fit has led many writers to suggest the moon as the source. The better-documented story is that several traditions with different reasons for using seven eventually merged. Which strand mattered most is debated, and the account below keeps to what is reasonably well supported.
Babylonian roots
Babylonian calendars were lunar, with months starting at the first sighting of the new crescent. Some Babylonian texts mark particular days of the month, including the 7th, 14th, 21st and 28th, as days with special rules or cautions: certain activities, such as travel or legal decisions, were to be avoided. Because these days fall a week apart, they are often described as a forerunner of the week.
There is a caution. These were days of the month, and the count restarted with each new moon, so a month of 29 or 30 days did not divide into complete sevens. This was not yet a week running continuously through the year. The Babylonians also gave the number seven a place in their cosmology through seven wandering bodies: the sun, the moon and five planets visible to the naked eye. That detail becomes important in the Roman period.
The Jewish week and the Sabbath
The biblical tradition set a seven-day cycle that did run without interruption. In the account of creation in Genesis, the work takes six days and the seventh is a day of rest. The commandment to keep the Sabbath then turns that account into a repeating practice. Whatever its earlier background, which scholars discuss at length, the Jewish week is a continuous count independent of the moon, with a fixed day of rest at the end of each cycle.
This is the point where the week becomes a rule. A Sabbath that comes every seventh day regardless of the month needs a continuous count of days, and that count was later carried on by Christian and Islamic communities, each choosing its own special day: Saturday, Sunday and Friday respectively. The creation story and its seven days are discussed elsewhere on the site, and the wider belief that seven is meaningful is discussed under seven in antiquity.
Roman markets and the planetary week
The Romans had their own short cycle, the nundinae, a market day that came every eighth day by their inclusive count. This was the rhythm of civic life in the republic, and it is a good reminder that eight-day and four-day cycles, like the four-day market week of the Igbo, were once at least as common as the seven-day one.
The planetary week arrived through Hellenistic astrology. Ancient astronomers ordered the seven wandering bodies by their apparent speed, from slowest to fastest: Saturn, Jupiter, Mars, the Sun, Venus, Mercury, the Moon. Astrologers then assigned the planets to the hours of the day in that repeating order, and the planet ruling the first hour of a day gave the day its name.
The arithmetic explains the strange order of the days. A day has 24 hours and 24 divided by 7 leaves a remainder of 3, so each new day begins three places further along the planetary list. Starting with Saturn, the first hour of the following days belongs to the Sun, then the Moon, Mars, Mercury, Jupiter and Venus. That yields the sequence Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, exactly the one we use.
This system shows how the twenty-four hour day, which the Egyptian decans contributed to, was needed before the planetary week could be calculated.
The days named for planets and gods
In Latin the days were named for the Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn, and the Romance languages preserve most of those names. French mardi, Spanish martes and Italian martedì all echo Mars. Germanic speakers translated the Roman gods into their own: Tuesday for Tiw, Wednesday for Woden, Thursday for Thor and Friday for a goddess usually identified as Frigg or Freyja. Saturday stayed Roman in English, and Sunday and Monday translated the Sun and Moon directly.
How the week spread
The planetary week was in use across the Roman world by the early centuries of the common era, alongside the Jewish week. In 321 CE the emperor Constantine issued a law making the “venerable day of the Sun” a day of rest from certain public business, which helped link the planetary week to the Christian Sunday. From there the seven-day cycle travelled with Christianity and later Islam, and with trade and colonial empires, until it became a global habit.
Beliefs about particular weekdays are another matter. A fear of an unlucky day, such as the one discussed under Tuesday the 13th, or the idea of lucky seven, is a folk tradition built on top of the calendar and says nothing about how the week began.
Putting the strands together
The week appears to have no single inventor. A lunar month with notable seventh days, a Jewish rest cycle running without a break, and a Hellenistic planetary scheme each contributed something, and the Roman Empire joined them. That is a reasonable picture, but the exact timing and the degree of mutual influence are matters on which historians still differ.