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How was the Order of the Days of the Week Determined?

The seven days of the week we use every day—Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday—are arranged in the same order week after week on the calendar. But who actually decided this order?

In fact, the names of the days of the week are deeply connected to celestial bodies like the Sun, Moon, and Mars. Furthermore, there was a mysterious idea held by ancient people that "each celestial body governs time every hour," which underlies the determination of this order.

The order of the days of the week can be explained by the ancient concept of rotating the seven celestial bodies every hour. Although it seems complicated at first glance, it's relatively simple when thought out numerically.

The Names of the Days of the Week Are Derived from Seven Celestial Bodies

First, let's take a look at the names from Sunday to Saturday. The names of the days in Japanese correspond to the Sun, Moon, and five planets that can be seen with the naked eye.

Day	Corresponding Celestial Body
Sunday	Sun
Monday	Moon
Tuesday	Mars
Wednesday	Mercury
Thursday	Jupiter
Friday	Venus
Saturday	Saturn

Thus, the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn are referred to as the "Seven Luminaries."

The "fire" in Tuesday corresponds to Mars, the "water" in Wednesday corresponds to Mercury, the "wood" in Thursday corresponds to Jupiter, the "metal" in Friday corresponds to Venus, and the "earth" in Saturday corresponds to Saturn. The kanji reveals the connection with each planet.

However, the celestial bodies are not arranged in this order. So, why is the order of the days of the week "Sun, Moon, Fire, Water, Wood, Metal, Earth"?

The System Behind the Determination of the Order of the Days of the Week

To understand the order of the days, it's necessary to know the arrangement of celestial bodies as conceived by ancient peoples and the system of assigning celestial bodies every hour.

1. Arranging Celestial Bodies by Distance

In ancient astronomy, Earth was considered the center of the universe, and there was a cosmological view that celestial bodies revolved around the Earth. Based on this idea, when arranging the seven celestial bodies by distance, it would be as follows:

Saturn → Jupiter → Mars → Sun → Venus → Mercury → Moon

This arrangement is not based on the current understanding of the distances between planets but reflects the cosmological views of that time.

2. Rotating Celestial Bodies Every Hour

In ancient astrology, a day was divided into 24 hours, and it was believed that each celestial body would govern each hour in turn.

If Saturn was in charge of the first hour, the next hour would be Jupiter, then Mars, and so on, repeating the aforementioned order. After reaching the seventh Moon, it would return to Saturn.

For instance, arranging the first nine hours would look like this:

Hour	Assigned Celestial Body
1st Hour	Saturn
2nd Hour	Jupiter
3rd Hour	Mars
4th Hour	Sun
5th Hour	Venus
6th Hour	Mercury
7th Hour	Moon
8th Hour	Saturn
9th Hour	Jupiter

It is crucial to note that the celestial bodies are repeated in sequence like this.

3. The Assigned Celestial Body Changes After 24 Hours

It is important to understand that a day has 24 hours.

If you divide 24 by 7, you have a remainder of 3. This means that after 24 hours, you will have gone through the seven celestial bodies three times and move three positions ahead.

If the first hour was Saturn, then 24 hours later, on the first hour of the next day, it will be Sun, which is three positions ahead of Saturn.

Saturn → Jupiter → Mars → Sun

That's why the day after a day ruled by Saturn is a day ruled by the Sun, or Sunday.

Continuing this same calculation, the next day after Sunday is Monday, and then Tuesday, leading to the current order of days of the week.

Understanding this mechanism reveals that the order of the days of the week was not arbitrarily established.

Why is There a Seven-Day Week?

So, why is there a seven-day week in the first place?

The seven-day cycle is said to have its origins in ancient Babylon. The phases of the Moon and the observation of celestial bodies were important, leading to the number seven being utilized as a unit of time. Subsequently, the seven-day week and the system of days named after celestial bodies spread throughout the ancient world.

However, it's necessary to consider separately the history of how the seven-day week originated and the history of how the order of days was defined by assigning the seven celestial bodies to specific times. The current system for naming days of the week is the result of the combination of these multiple historical influences.

When Was the Concept of the Seven Luminaries Introduced to Japan?

The idea of the seven luminaries was introduced to Japan during the Heian period. According to explanations from the National Astronomical Observatory, the "Shukuyōkyō" (a text attributed to Kōbō Daishi, Kūkai) records the seven luminaries corresponding to the Sun, Moon, and five planets.

At that time, the days of the week in Japan were used not so much for managing work and school schedules as they are today; rather, they were utilized for purposes like divining the auspiciousness of particular days. The recording of days of the week in calendars gradually led to their acceptance in Japan.

Later, during the Meiji period, Japan adopted the Western solar calendar. The system designating Sunday as a day off was established during the Meiji period, strengthening the connection between the current days of the week and daily life.

English Days of the Week Also Have Celestial Relationships

The relationship with celestial bodies is not only present in Japanese but also remains in the names of the days of the week in English.

Day	English	Derived Celestial Body/God
Sunday	Sunday	Sun
Monday	Monday	Moon
Tuesday	Tuesday	God Tyr from Norse mythology
Wednesday	Wednesday	Related to the god Odin from Norse mythology
Thursday	Thursday	God Thor from Norse mythology
Friday	Friday	Related to a goddess from Norse mythology
Saturday	Saturday	Saturn

It is evident that Sunday and Monday originate from the names for the Sun and Moon. Meanwhile, from Tuesday to Friday, the celestial bodies from Roman mythology were replaced by deities from Germanic and Norse mythology.

It is intriguing to see the varying ways in which different languages name the same day.

Understanding the Origins of the Days of the Week Makes Our Familiar Calendars More Interesting

Though we typically use the days of the week without much thought, their background is influenced by ancient astronomy, astrology, religion, and the culture of various regions.

The names of the days correspond to the seven celestial bodies, and by utilizing the concept of assigning celestial bodies every hour, the reasons for the order of Sun, Moon, Fire, Water, Wood, Metal, Earth can be explained.

Of course, modern science does not consider celestial bodies to govern time. This concept reflects historical thinking about how people of that time understood the world and the universe.

Once you learn about the origins, the seven days listed in the calendar might seem slightly different to you.

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