I was looking for information on the kanji 曜 and I ran across this explanation.

日・月と五星(火・水・木・金・土)のの総称(七曜)」日本のみで用いられる意味 Sun, moon, and five stars (fire, water, tree, metal, earth) term for seven days used only in Japan.

This appears to be a story of some kind. I'd like to know if there is a myth or other interesting saga behind these names. I'll looking elsewhere, but the kind people on this site seem to find such answers much more quickly.


I think I had a small breakthrough. The days of the week in Japanese and the names of the planets correlate. The "five stars" should have been "five planets", but I was too dense to know.

Okay, so that means Tuesday-Saturday is Fire, Water, Wood, Metal, Earth which is also Mar, Mercury, Jupiter, Venus, Saturn. Now, I know enough European languages to know that the order is based on the same planet-named days from French/Spanish/English. If the days of the week were originally in this order in Japanese, it would be highly unusual. A big coincidence.

It looks to me like the names of the days from some european language(s) were used and the Japanese just assigned the equivelent word from their language. I can almost hear some Frenchman saying lundi or the equivalent Monday/lunes in ENglish/Spanish and someone in Japan saying "okay if you really want to call it moon-day".

------------------update------------------------ I found this "It is believed that the Chinese words for fixed stars, planets, and long stars were all adopted from ancient Chinese cosmology, which was referred to when Western European astronomical texts were translated into Chinese by Mateo Ricci and his collaborators in the late Ming and early Qing dynasties." https://ja.wikipedia.org/wiki/%E6%81%92%E6%98%9F

So, it appears that someone did translate the Latin planet-names into Chinese. Again, I'd be very surprised that the names correlated so specifically with day-names, but it would make sense if they translated the already translated latin planet-names for day-names.

------------Final Update--------------------- Apparently, the original Japanese concept for the division of a month was three ten-day periods 上旬 - First 10 days 中旬 - Middle 10 days 下旬 - Last 10 days

The Latin-Chinese (and thus Latin-Japanese) names for celestial bodies were known by the end of the 1600s and when the 7-day week was introduced, Japanese speakers adopted a translation of those celestial bodies assigned to days of the week. It was a European concept and they just used European names. This is similar to the way that Norse week-names were adopted into English.

This leaves me with three questions:

  • Does anybody have historical insight into this? Am I guessing correctly? When did the Japanese adopt the sun-moon-planets model?
  • Did the Japanese have their own names for the days of the week before this happened?
  • Maybe it's been going on for so long that modern Japanese don't notice, but is there some sense that they are using a foreign naming system for days?

----------REALLY final update I promise------------

This question was closed after I got my answers and it's a bit frustrating that it's being linked with a question that says they were named after the planets and then some western influence. Just not enough. After all the help received below, I wrote up my own aswer as follows.

Before we get to Japanese, let's talk about how English got its days of the week. In Latin, the days of the week were Sun-day, Moon-day, Mars-day, Mercury-day, Jupiter-day, Venus-day, and Saturn-day. They literally took the names of the sun, moon, and five visibile planets to give names to the days. In many Latin-based languages Saturn-day was changed to Sabbath. Also, the Sun-day was changed to Lord's Day. In Spanish they are sabado and domingo. In French it is samedi and dimache. These changes were made because of the strong influence of Christianity. English had similar influences from Latin, but the Norse influence kept Saturday and Sunday the same, while changing Ty's-day, Ven's-day, Thor's-day, and Fir's-day. These were all names of their gods, so it didn't really change the concepts.

Over in China and Japan, they had divided their 30-day months into 3 decamerons--10 day periods. They called them 旬 (じゅん) or seasons (NOT spring, summer, fall, or winter). They named them early season 上旬, middle season 中旬, and late season 下旬. Each of the seasons were further divided into two 5-day periods with each day associated with an element.

The Chinese and Japanese saw the same planets that the Romans did. They didn't think about them as gods, but they did associate them with the same elements we have already learned. Mars (red and firey to the Romans) was also 火星 (かせい) fire planet to the Chinese. Unfortunately the names of the planets are different from the elements themselves. 火 should be ひ, but the planet is かせい not said ひせい. This is something like the way that the topic marker は is pronounced 'wa' instead of 'ha'. Same character, different meaning and sound.

Around 1600, there was a Catholic missionary, Matteo Ricci, who went to China and stayed there the rest of his life. He got to meet the emperor and taught many people out of the Latin books he brought with him. It is believed that the Chinese words for planets were all adopted from ancient Chinese cosmology mixed with the Latin cosmology translated into Chinese by Ricci.

The Chinese already had elements associated with the planets. Now the Latin calendar added a day of the week for each planet, the sun, and the moon. So, the Chinese were quite happy to simply adopt all the Chinese translations of the Latin day names. They call them 曜 (よ) and sometimes the Japanese refer to them as the "sun, moon, and five planets". Now, the Japanese use the same 7-day week common in Europe and the names exactly correlate with the Latin calendar. The Chinese gave it all up and just call them one-day, two-day, etc.

  • 1
    Are you asking about this? Or how they came to be used to refer to the days of the week in Japan?
    – aguijonazo
    Sep 4, 2022 at 16:03
  • @broccolifacemask Thank you but no. I'm not entirely clear on the distinction in that question, but I'm not asking about 曜 now as much as the explanation of the names. What I read suggested the days of the week were named after something mythological and it now appears there is an exact correspondence to the Latin-based order common to European languages. The answer to the first part of my question was that "five stars" really means "five days".
    – BSD
    Sep 4, 2022 at 16:29
  • 1
    I think the division of days into seven day weeks is relatively recent historically. I think in the past the Japanese used 10 day periods, called decameron in English or more easily 旬.
    – Ragaroni
    Sep 4, 2022 at 16:42
  • 1
    This could help you, too: sljfaq.org/afaq/days-of-week.html Sep 4, 2022 at 21:24
  • 1
    火星 is Kasei and not Hisei not because of sound change(like は), but simply because it’s 音読み of 火, not 訓読み. I’m English terms, ka is the Chinese reading of 火, whereas hi is the Japanese native reading of the same character. 火星 being a kango word, is read in 音読み。
    – dvx2718
    Sep 7, 2022 at 21:14

1 Answer 1


The division of days into seven day weeks is relatively recent historically. I think in the past the Japanese used 10 day periods, called decameron in English or more easily 旬.

So, the Japanese must have nore or less adopted the Western week system, which is why we see so many similarities.

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