Why the week has seven days, and where weekday names come from
Babylonians, Jewish tradition and the seven naked-eye wanderers of the sky all shaped the week. Here is what is fact, what is hypothesis, and why Portuguese weekdays are numbered.
The day comes from Earth's rotation, the month is tied to the Moon and the year follows our trip around the Sun. The week is different: no astronomical cycle lasts exactly seven days. Even so, almost the entire world runs on blocks of seven. The explanation blends Mesopotamia, Jewish tradition, Greco-Roman astrology and a church choice that gave Portuguese its numbered weekdays.
Babylon and the phases of the Moon
The oldest records of a seven-day rhythm come from Mesopotamia. In the Babylonian calendar, days 7, 14, 19, 21 and 28 of each month were special, associated with restrictions and rest. Notice that, except for the 19th, they are multiples of seven, and that the gaps closely follow the phases of the Moon: new, first quarter, full and last quarter.
The lunar link is the most popular explanation for the number seven, but it has a flaw: a lunar cycle lasts about 29.5 days, not 28. Also, the Babylonian rhythm restarted every month; it was not yet the continuous week we use.
Jewish tradition and the Sabbath
The continuous week was established by Jewish tradition. In the biblical account of Creation, the world is made in six days and the seventh is a day of rest, the Sabbath. The days were simply numbered, except for the Sabbath. Major encyclopedias note that the Jews may have taken the idea of a seven-day cycle from Mesopotamia, possibly through contact with Babylon, but this remains a hypothesis. Christianity inherited this week and gave pride of place to Sunday, the day of the Resurrection.
Seven wanderers, seven days
The third piece comes from ancient astronomy. With the naked eye, people knew seven bodies that move against the background of the stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. Astrologers in the Hellenistic world linked each day to one of them, and the Romans adopted this planetary week. In 321, Emperor Constantine made rest on the day of the Sun official.
The order of the days has a curious explanation, recorded by the Roman historian Cassius Dio in the 3rd century: the bodies were ranked from slowest to fastest (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon) and each ruled one hour of the day in rotation. Whichever ruled the first hour gave the day its name. Counting through all 24 hours, Saturn's day is followed by the Sun's, then the Moon's, and so on. Dio himself presents this as a tradition, not a certainty.
| Body | Latin | Spanish | English | Japanese |
|---|---|---|---|---|
| Sun | dies Solis | domingo | Sunday | nichiyobi |
| Moon | dies Lunae | lunes | Monday | getsuyobi |
| Mars | dies Martis | martes | Tuesday | kayobi |
| Mercury | dies Mercurii | miércoles | Wednesday | suiyobi |
| Jupiter | dies Iovis | jueves | Thursday | mokuyobi |
| Venus | dies Veneris | viernes | Friday | kinyobi |
| Saturn | dies Saturni | sábado | Saturday | doyobi |
English brought in equivalent Germanic gods: Tiw, Woden, Thor and Frigg. Japanese, which made its names official in 1876, uses the Sun, the Moon and the elements linked to the planets: fire, water, wood, metal and earth.
Why Portuguese weekdays are “feiras”
The feira in segunda-feira (Monday) has nothing to do with markets. It comes from the Latin feria, a word that in Church Latin came to mean the days of the liturgical week, counted from Sunday: feria secunda, feria tertia and so on. In the 6th century, Martin of Dumio, bishop of Braga, criticized Christians who still named the days after pagan gods and championed the church form.
He did not invent the system, which already existed, but his preaching helped root it in the region where Galician-Portuguese would emerge. That is why Portuguese is the main Romance language that dropped the planetary names. Saturday and Sunday were left out: sábado comes from the Hebrew shabbat and domingo from the Latin dominicus, “of the Lord”.
What is fact and what is hypothesis
- Fact: the Babylonians marked special days at intervals close to seven within each month.
- Fact: the continuous week with a day of rest took hold in Jewish tradition and passed to Christianity.
- Fact: the planetary names come from the seven naked-eye bodies and were spread by the Romans.
- Hypothesis: the number seven was chosen because of the phases of the Moon.
- Hypothesis: the Jewish week was directly inspired by Babylon.
- Ancient tradition, unconfirmed: the order of the days comes from the hourly rotation of the planets.
In the end, the seven-day week is a human invention that combined sky-watching, religion and habit. It even survived the Gregorian reform of 1582, which skipped ten dates without touching the order of the days. When you say Friday, you honor a Germanic goddess; a Spanish speaker, on the same day, honors Venus.
