The Sky Ordered Our Week: Monday, Tuesday, Wednesday…

The days of the week: Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn — Representation of the seven ancient "planets" (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn) in the order that determined our week.
Image source: astronoo.com (new window)

Scientific Summary

The order of the days of the week (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday) originates from an ancient astrological rule called the "rule of planetary hours." The Babylonians established an order of the "wandering stars" (planets) from slowest to fastest: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. By assigning each hour of the day to a planet in this cyclical order, the first hour of the first day fell to Saturn (Saturday), and the first hour of the following day to the Sun (Sunday), then to the Moon (Monday), etc., thus generating the sequence we know.

Why are the days of the week in this precise order: Monday, Tuesday, Wednesday…?

The order of the days of the week comes from an ancient astrological rule called the "rule of planetary hours." The Babylonians established an order of the "wandering stars" (planets) from slowest to fastest: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. By assigning each hour of the day to a planet in this cyclic order, the first hour of the first day fell to Saturn (Saturday), and the first hour of the next day to the Sun (Sunday), then to the Moon (Monday), etc., thus generating the sequence we know.

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From planet to modern day

This table shows how the names of the days, derived from the same Latin astrological background, evolved differently across language families.

The names of the days across languages: an astral and divine heritage
Governing star	Latin origin	French (Romance language)	Spanish (Romance language)	English (Germanic language)	Etymology
☉ Sun	Dies Solis	Dimanche	Domingo	Sunday	From Latin dominicus (Lord's day). English retains the solar reference.
☾ Moon	Dies Lunae	Lundi	Lunes	Monday	Directly derived from the Latin name for the Moon (Luna).
♂ Mars	Dies Martis	Mardi	Martes	Tuesday	From the Roman god Mars. English comes from Tiw (Germanic god of war).
☿ Mercury	Dies Mercurii	Mercredi	Miércoles	Wednesday	From the Roman god Mercury. English comes from Woden (Odin, supreme Germanic god).
♃ Jupiter	Dies Jovis	Jeudi	Jueves	Thursday	From the Roman god Jupiter/Jove. English comes from Thor, its Nordic equivalent.
♀ Venus	Dies Veneris	Vendredi	Viernes	Friday	From the Roman goddess Venus. English comes from Frigg or Freya, Nordic goddesses of love.
♄ Saturn	Dies Saturni	Samedi	Sábado	Saturday	French and Spanish derive from the Hebrew Sabbat. English retains the reference to Saturn.

The "Chaldean" order of the planets: a hierarchy based on observation

The key to this order lies in the observation of the ancient sky. Mesopotamian, then Greek and Roman civilizations identified seven "wandering stars" visible to the naked eye, which they called "planets": the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. According to the geocentric cosmology of the time, it was believed that these celestial bodies exerted an influence on human affairs.

A succession of days inherited from a learned tradition

The established order of our days (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday) is so familiar to us that it seems natural. Yet, this succession is not the result of chance, but the product of a learned tradition over two millennia old, blending Babylonian astronomy, Roman mythology, and astrological calculations.

The Chaldean order and the criterion of apparent speed

The so-called "Chaldean" order of the planets (Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon) was established by Babylonian astronomers between the 8th and 6th centuries BCE. It was determined by the apparent orbital periods around the Earth, from longest to shortest, according to the geocentric model of the time. This order of "presumed slowness" classified the planets as follows: Saturn (≈ 29.5 years, the slowest), Jupiter (≈ 11.9 years), Mars (≈ 687 days), Sun (≈ 1 year), Venus (≈ 224 days), Mercury (≈ 88 days), Moon (≈ 27.3 days, the fastest).

Transmission in the Greco-Roman world

This order was then adopted by the Greeks, then systematized by Roman scholars such as Ptolemy (2nd century) for astrology and the calculation of planetary hours.

The rule of "planetary hours"

Each hour of the day and night was governed by a planet in a perpetual cycle, influencing, according to beliefs, the auspicious moment to undertake any action.

A cycle based on the planetary order

This attribution followed a precise cycle based on the supposed order of the planets. The first hour was governed by Saturn (the slowest planet), then a cyclical sequence was applied.

The calculation mechanism

Saturn ruled the first hour of the day. The 8th, 15th, and 22nd hours also returned to Saturn. In order of "slowness", the 23rd hour was governed by Jupiter, and the 24th hour was governed by Mars. The first hour of the following day therefore returned to the Sun.

From hour to day to our modern week

By continuing this logic, we obtained the order of the days as we know them. The entire day took the name of the planet that governed its first hour. Thus was born the sequence: Saturn (Saturday), Sun (Sunday), Moon (Monday), Mars (Tuesday), Mercury (Wednesday), Jupiter (Thursday), Venus (Friday).

The astrological mechanics that fixed our week

If we start the cycle with Saturn (1st hour of the 1st day), then by applying the 24-hour rule, we obtain the ancient/original order of the days as used in Roman times.

How the cycle of hours creates the order of the days
Day Latin name	1st hour → "Master of the day"	Following hours (cyclical sequence)	24th hour (last)
Saturday Dies Saturni	♄ Saturn	♃ → ♂ → ☉ → ♀ → ☿ → ☾ → ♄ → …	♂ Mars
Sunday Dies Solis	☉ Sun (next in order after Mars)	♀ → ☿ → ☾ → ♄ → ♃ → ♂ → ☉ → …	☿ Mercury
Monday Dies Lunae	☾ Moon (next after Mercury)	♄ → ♃ → ♂ → ☉ → ♀ → ☿ → ☾ → …	♀ Venus
Tuesday Dies Martis	♂ Mars (next after Venus)	☉ → ♀ → ☿ → ☾ → ♄ → ♃ → ♂ → …	♃ Jupiter
Wednesday Dies Mercurii	☿ Mercury (next after Jupiter)	♀ → ☾ → ♄ → ♃ → ♂ → ☉ → ☿ → …	☾ Moon
Thursday Dies Jovis	♃ Jupiter (next after Moon)	♂ → ☉ → ♀ → ☿ → ☾ → ♄ → ♃ → …	♄ Saturn
Friday Dies Veneris	♀ Venus (next after Saturn)	☿ → ☾ → ♄ → ♃ → ♂ → ☉ → ♀ → …	☉ Sun

Note: The sequence of planets in the cycle is always the same: ♄ Saturn → ♃ Jupiter → ♂ Mars → ☉ Sun → ♀ Venus → ☿ Mercury → ☾ Moon → (then it starts again). The apparent "jump" between the master of one day and that of the next is due to the 24 intervening hours.

What is the first day of the week?

There is no natural first day of the week. The placement of Sunday or Monday at the head of the weekly cycle is a matter of cultural and religious conventions. Sunday long held this position in the West due to its importance in biblical and Christian tradition, institutionalized in the time of Constantine. Conversely, the ISO 8601 standard, adopted in 1988, sets Monday as the first day of the week in a secular and administrative context.

There is no first day of the week, but two systems
First day	Last day (weekend)	Global coverage	Basis
Monday	Sunday	~65% of the world population (Europe, Asia, Africa, South America)	ISO Standard (organizational logic)
Sunday	Saturday	~30% of the world population (North America, Australia, Israel, Philippines)	Religious tradition (Judeo-Christian heritage)

FAQ: Everything about the origin of the order of the days of the week

What is the "Chaldean order" of the planets?

The Chaldean order is a sequence of the seven "wandering stars" (planets) established by Babylonian astronomers between the 8th and 6th centuries BCE. It was based on their apparent speed around the Earth in the geocentric model, from slowest to fastest: Saturn (~29.5 years), Jupiter (~11.9 years), Mars (~687 days), Sun (~1 year), Venus (~224 days), Mercury (~88 days), Moon (~27.3 days). This order was transmitted to the Greeks and then to the Romans.

How did the "rule of planetary hours" fix the order of the days?

Ancient astrologers divided the day into 24 hours (day and night). Each hour was governed by a planet following the Chaldean order in a perpetual cycle. The planet that governed the first hour of a day gave its name to that entire day. Starting with Saturn (1st hour of the 1st day), the 24th hour fell to Mars. The first hour of the next day therefore fell to the Sun (Sunday). Continuing the calculation gives the sequence: Saturday (Saturn), Sunday (Sun), Monday (Moon), Tuesday (Mars), Wednesday (Mercury), Thursday (Jupiter), Friday (Venus).

Why does the first day of the week vary between cultures?

There is no natural first day of the week. Sunday is placed first in cultures with Judeo-Christian tradition (about 30% of the world population, including North America, Australia, and Israel) because it is the biblical day of rest. ISO 8601 standard (adopted in 1988) sets Monday as the first day for organizational and secular reasons; it is the standard used in Europe, Asia, Africa, and South America (about 65% of the world population).