Weekdays in Japanese hiragana

Japanese Days of the Week: the 'Seven Luminaries'

The Japanese days of the week seem quite different from anything in Western languages:

(For the pronunciation of these names, see Youtube videos A B C D. Do not listen to videos that do not use native speakers.)

The initial impression of these names is one of primitivity and mystery. The first Chinese character in the name of each day is a simple concept linked with an elemental force of nature such as fire, wood, water, etc. The final character 日 hi, here pronounced bi, means 'sun' or 'day'. The middle character 曜 yō, though familiar to Japanese speakers through its use in the days of the week, is relatively obscure. Yōbi (曜日) is, to all intents and purposes, a single expression meaning 'day of the week'.

Yet the key to understanding the names of the days of the week lies in that obscure second character 曜 yō. Pronounced yào in Mandarin Chinese, 曜 means 'sunlight' or 'luminary; shining body'. The sun, the moon, and the five planets were called the 'seven luminaries' (七曜 Japanese shichi-yō, Mandarin qī yào) by the ancient Chinese, much as the ancient Greeks and Romans referred to them as the seven planets. It is from the 'seven luminaries' that the Japanese names of the days of the week are derived.

The first two days are no mystery. They are named after the sun and the moon. To understand the remaining five we must look at the ancient Chinese theory of the 'Five Elements' 五行 (Japanese go-kō, Mandarin: wǔ-xíng). The Five Elements started out as a primitive system for explaining the universe but gradually developed to become an all-embracing cosmological system. Each element was equated to (among others) a direction, a colour, a season, a time of day, a planet, and a musical note in the pentatonic scale.

Fire (火 Japanese: ka, Mandarin: huǒ) was equated to south, red, summer, midday, the planet Mars, and note 5 in numbered musical notation.
Water (水 Japanese: sui, Mandarin: shuǐ) was equated to north, black, winter, midnight, the planet Mercury, and note 6 in numbered musical notation.
Wood (木 Japanese: moku, Mandarin: mù) was equated to east, green, spring, dawn, the planet Jupiter, and note 3 in numbered musical notation.
Gold or Metal (金 Japanese: kin, Mandarin: jīn) was equated to west, white, autumn, dusk, the planet Venus, and note 2 in numbered musical notation.
Earth (土 Japanese: do, Mandarin: tǔ), was equated to the centre, yellow, 18 days at the end of each season, the planet Saturn, and note 1 in numbered musical notation.

The first five planets of the solar system in Chinese, Japanese, and Vietnamese are still named after the Five Elements. In Chinese and Japanese, Mercury is called 水星 ('Water Star'), Venus is called 金星 ('Metal Star'), etc. Vietnamese puts the word Sao ('star') in front of each element to form the name of the planet. (Note 2: Names of the outer planets & earlier planet names)

So the Japanese days of the week are not a system of 'primitive elements' after all! In fact, they run in parallel with days of the week of the Ancient Greeks and Romans more closely in parallel, indeed, than modern English:

How did the Japanese get these names?

The obvious question is: could there be a historical link between the 'seven luminaries' and the 'seven planets' of Western and Middle Eastern antiquity?

The answer is, yes. The seven-day week appears to have originated in ancient Mesopotamia or Egypt. Who first named these days after the planets is not totally clear. It was possibly the Greeks, who passed it on to the Romans. This naming found its way to India and then to China. However, the specific route and timing is not clear. The Cihai (辞海), a Chinese encyclopaedia, carries the following entry for 七曜历 qī yào lì (Japanese 七曜曆 shichi-yō-reki ), or 'seven luminaries calendar':

七曜历 qī yào lì, i.e., method of recording days according to the 七曜 qī yào. China normally observes the following order: sun, moon, Mars, Mercury, Jupiter, Venus, and Saturn. Seven days make one week, which is repeated in a cycle. Originated in ancient Babylon (or ancient Egypt according to one theory). Used by the Romans at the time of the 1st century AD, later transmitted to other countries. This method existed in China in the 4th century. It was also transmitted to China by Manichaeans in the 8th century from the country of Kang (康) in Central Asia (Note: The country of Kang).

The earliest use of the 'seven luminaries' (七曜 qī yào / shichi-yō) is attributed by Cihai to Fan Ning (範寧 / 范宁, Japanese Han Nei), a scholar who lived from AD 339-401. Tellingly, the Chinese 'seven luminaries' were arranged in the same order as the Indian planetary names for days of the week, not in the classic order of the Chinese five elements, which put water before fire.

Besides the Manichaean route noted by the Cihai, there was also an Indian route of transmission in the 8th century. The Chinese Buddhist monk Yi Jing (義凈 / 义净 Japanese Gi Jō) and the Ceylonese or Central Asian Buddhist monk Bu Kong (不空, Japanese Fu Kū, also known as Amoghavajra) are both credited with referring to the seven-day cycle of planetary names in their writings, drawing on Indian sources. The Indians, in turn, appear to have taken this from the West. (Note: The Buddhist route of transmission)

Although there were several routes of transmission into China, it appears that the Indian route was the direct source of the Japanese names for days of the week. In 806, the famous Japanese monk, Kobo Daishi (弘法大師) (Note: Kobo Daishi) brought Bu Kong's writings back to Japan along with a huge quantity of other Buddhist scriptures. Great interest was taken in Bu Kong's astrological work by Japanese astronomers, with the result that the planetary names found their way into Japanese calendars of the time. One such calendar was used by the Japanese statesman Fujiwara no Michinaga (藤原道長) for writing his diary in 1007, in which the present-day Japanese names for the days of the week can be found (Note 6: Fujiwara no Michinaga).

Although not in widespread use except for astrological purposes, this system of names was nevertheless maintained by the Japanese right through to the modern era. At one stage the days got out of kilter in eastern Japan and had to be rectified by a calendar reform in 1685. When they came under pressure to harmonise their working calendar with the West in the latter half of the 19th century, the Japanese turned to this old system to name the days of the week, officially adopting them in 1876. After this the names gradually came into general use in Japan (Note: The crucial step).

China also maintained the planetary names at least as late as the 19th century. Indeed, the Chinese adopted the word 星期 xīngqī 'star period', to mean 'week' by reference to the planetary names. But when the seven-day week was adopted as part of the Westernisation (or "modernisation") of Chinese life in the modern era, the Chinese turned to a completely different, home-grown system to name the days of the week.

Incidentally, the Japanese word for 'week', 週 shū, is etymologically derived from Chinese roots and has the meaning of 'cycle'. It has since been borrowed back into Chinese as one of the alternative words for 'week'.