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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.

---------------update--------------------

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](https://en.wikipedia.org/wiki/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.
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Why do days of the week in Japanese (Sun, Moon, and Five Planets) exactly match Latin-based calendars?
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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