Where did the Japanese get their names for the weekdays?

	Japanese weekday (romaji)	In kanji	Kanji meaning	Planet	Planet in Japanese
Monday	Getsuyōbi	月曜日	The moon	The moon	Tsuki (月)
Tuesday	Kayōbi	火曜日	Fire	Mars	Kasei (火星)
Wednesday	Suiyōbi	水曜日	Water	Mercury	Suisei (水星)
Thursday	Mokuyōbi	木曜日	Wood; tree	Jupiter	Mokusei (木星)
Friday	Kin'yōbi	金曜日	Metal; gold	Venus	Kinsei (金星)
Saturday	Doyōbi	土曜日	Earth, soil	Saturn	Dosei (土星)
Sunday	Nichiyōbi	日曜日	Sun	The sun	hi (日); taiyō (太陽)

Until opening up to Western influence, the Japanese used a lunar calendar with no weeks, and called the days of the month by basically numerical names (tsuitachi, futsuka, ... misoka) (see What are the Japanese days of the month?). Yet immediately upon adopting the Western calendar, they had names for the seven days of the week. Where did they come from?

The words for Sunday and Mo(o)nday are "'sun'-yōbi" and "'moon'-yōbi" respectively. This should be a clue that they are translations of Western names.

The Romans (translating the Greek, presumably) named the days after seven celestial luminaries, the sun, the moon, and the five readily visible planets. The planets had the names of certain gods, so the result was names that would translate into English as "Sun day, moon day, Mars day, Mercury day, Jupiter day, Venus day, Saturn day". Except for "Saturn day" and "Sun day," which suffered the effects of Christianity, the days of the week in the Romance languages still reflect those origins. In English we have, for Tuesday through Friday, names based on more-or-less equivalent Norse gods, with Satur(n) apparently filling a blank.

The Chinese had named (renamed?) the "naked-eye" planets after the Five Elements, water, metal, fire, wood, and earth, so Mercury was "water star", Venus "metal star", Mars "fire star", Jupiter "wood star", and Saturn was "earth star". The Japanese renderings of the Chinese names are, in the same order, kasei (火星), suisei (水星), mokusei (木星), kinsei (金星), and dosei (土星).

The Chinese also distinguished the "Seven Luminaries" from among the luminaries 曜 (on-yomi エウ = ヨウ) in the heavens. They included nichiyō (日曜) (the solar luminary), getsuyō (月曜) (the lunar), and the five planets with "-yō" replacing "-sei" ("star") in their names. The names of these Seven Luminaries were obvious translations for the original Western weekday names.

Who translated the names, and when? This seems to be a bit of a mystery. Perhaps compounding the mystery is the fact that no common Western language preserves the "luminary"-to-weekday correspondence intact for all seven days.

Disclaimer: Nothing in the above is intended to mean that the Greeks invented the names of the days, or even that it happened first in the West.

Acknowledgements

Contributed by Bart Mathias.

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