1. Why the "Earth" in Saturday Means a Star, Not Soil
In Japanese, the days of the week correspond to specific celestial bodies: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. Together, these seven are called the "Seven Luminaries" or *shichiyou*. [1] While planets have other traditional names in Japan, such as *Saishou* for Jupiter or *Taibai* for Venus, the names used for the days of the week are linked to the Five Elements theory (Wood, Fire, Earth, Metal, Water). It is said that these elemental names were chosen to translate the foreign astronomical terms. [1] The old name for Saturn in Japanese was *Chinsei*, so the "Earth" day is indeed the day of this planet.
This raises a curious question: how did these seven stars become arranged into a single week? Who brought this specific order to Japan, and when did it arrive? [1]
2. How Kukai and Michinaga's Diary Record the Seven Luminaries in Japan
It is said that during the Heian period, the monk Kukai (also known as Kobo Daishi) brought the Seven Luminaries to Japan along with a text called the *Sukuyokyo*. Kukai lived from 774 to 835, and some records explain that he returned from Tang China in the early 9th century. [3] Evidence of this system appears in the diary of Fujiwara no Michinaga, a national treasure called *Midokanpakuki*. This diary includes days of the week—Sun, Moon, Fire, Water, Wood, Metal, Earth—recorded in a calendar made by *Onmyoji* (diviners) called the *Guchureki*. [3] For example, the date recorded as the 19th day of the 2nd month of the 6th year of Choho (which corresponds to March 12, 1004 in the modern calendar) is noted as a Sunday, and modern calculations confirm it was indeed a Sunday. [3] Studies of calendars from the Kamakura to Edo periods, including the years 1210, 1315, 1688, and 1813, report that the days of the week for these years align with the current system. [3]
However, these days were not weekends in the modern sense. According to the *Sukuyokyo*, the Seven Luminaries were used with the Twenty-Eight Mansions to divine the fortune of the day. Sunday became a holiday in April 1876 (Meiji 9). [1] So, who decided the logic behind the order "Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn"? [1]
3. How Giving Each Star One Hour, Starting From the Farthest, Sets the Weekday Order
The background to this order lies in the geocentric model, which places Earth at the center. In this view, celestial bodies are arranged by distance from Earth, starting with the farthest: Saturn, Jupiter, Mars, Sun, Venus, Mercury, and Moon. [2] One theory for the order of the days comes from astrology. It assumes that celestial bodies rule time. If you assign the first hour of the day to Saturn, the second to Jupiter, the third to Mars, and so on in order of distance, the 24th hour falls to Mars. The next day’s first hour then falls to the Sun. If the body ruling the first hour rules the whole day, the sequence becomes Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus. [2] Note that the table using "1 to 24 hours" is a modern rewriting for explanation.
The reason for this shift is division. A day has 24 hours, and there are 7 stars. When you divide 24 by 7, the remainder is 3. This means that at the end of each day, the starting star moves forward by 3 places in the sequence of 7. Counting Saturn as the start, moving 3 places forward lands on the Sun. Moving 3 more lands on the Moon. After 7 days, the sequence returns exactly to Saturn. [2] But who explained this, and in which documents?
4. A Music Theory Explanation for the Weekday Order, and Why the Truth Is Unclear
In the 3rd century, the historian Cassius Dio wrote in his *Roman History* that there are two explanations for the order of the days. One is the "one hour per planet" method described above. The other is a music theory explanation, where picking celestial bodies every fourth one according to the tetrachord theory of music yields the order of the days. The National Astronomical Observatory of Japan states that it is unknown which theory is correct. [2]
Cassius Dio wrote that the custom of linking days to the seven planets began with the Egyptians. He noted that while it is now universal, its adoption was relatively recent. [4] The habit of naming planets after gods started with the Sumerians and passed to the Babylonians, Greeks, and Romans. Each time it traveled, the names were replaced with the gods of that culture’s mythology. [1] How did these names reach English?
SAFETY NOTE: This habit traveled into languages outside of Rome, such as English. It is interesting to see which days kept the god names and which did not.
5. Why English Saturday Means Saturn's Day and Was Not Renamed for a God
The English word *Saturday* comes from Old English *sæterdæg* or *sæternesdæg*, meaning "Saturn’s day." This was a partial translation of the Latin *Saturni dies* (Day of Saturn), which itself translated the Greek "Day of Cronus." [5] In English, the days from Tuesday to Friday were replaced by Norse gods instead of Roman ones. Mars became Tiw’s day (Tuesday), Mercury became Woden’s day (Wednesday), Jupiter became Thor’s day (Thursday), and Venus became Freya’s day (Friday). [1] However, Saturday was not replaced. Etymology dictionaries speculate that this might be because there was no Norse god equivalent to the Roman Saturn. This is a guess, not a confirmed reason. [5]
Other languages have different roots. German *Samstag*, French *Samedi*, and Spanish *sábado* come from a Hebrew word for the Jewish Sabbath, not from planets. Languages that use planet names for days include Japanese, as well as those in India, Thailand, and South Korea. [1] How far can you verify this yourself?
6. How to Write a 24-Hour Table on Paper to See Why the Week Starts With Saturday
You can reproduce this calculation with just paper and pencil. Write the seven stars in order of distance: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Below them, write the stars again, repeating the sequence for hours 1 through 24. Hour 24 should fall to Mars. The very next hour, hour 25 (which is hour 1 of the next day), becomes the name of that day. If you continue this for 7 days, you will see the starting star move in the order Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus, and return to Saturn after 7 days. [2]
Once you have written it, compare it with the day-of-the-week order on your home calendar. If the starting point is off, check how you counted the sequence of 7 stars. For further research, the National Astronomical Observatory’s calendar Wiki contains the correspondence table for the Seven Luminaries and the two explanations for the order. [1][2] You can also look at calendars from other countries to see if the name for Saturday comes from a planet or from the Sabbath.