Youbi (曜日) & Shichiyou (七曜): Meaning in Japanese

Recently, I have learned from Japanese Wikipedia that the names of the days of the week or Youbi (曜日: meaning Day of the Week), that is,

1. Nichi-Youbi (日曜日: Sunday)
2. Getsu-Youbi (月曜日: Monday)
3. Ka-Youbi (火曜: Tuesday)
4. Sui-Youbi (水曜日: Wednesday)
5. Moku-Youbi (木曜日: Thursday)
6. Kin-Youbi (金曜日: Friday)
7. Do-Youbi (土曜日: Saturday) comes from Shichiyou (七曜).

What is Shichiyou in Japanese?

Originated from ancient Chinese astronomy, Shichiyou (七曜) [*] is the word for the seven celestial bodies,

1. Taiyou (太陽: Sun)
2. Tsuki (月: Moon)
3. Ka-Sei (火星: Mars)
4. Sui-Sei (水星: Mercury)
5. Moku-Sei (木星: Jupiter)
6. Kin-Sei (金星: Venus)
7. Do-Sei (土星: Saturn)

* In the Japanese term, 七曜 (Shichiyou), 七 (Shichi) is the Kanji character for 7, while 曜 (You) refers to luminaries.

Youbi based on Shichiyou

As you can notice by now, Youbi is based on Shichiyou, and it is believed that

1. Nichi-Youbi (日曜日: Sunday) is the day guarded by Ohisama (お日様: Sun) [*]
2. Getsu-Youbi (月曜日: Monday) by Tsuki (月: Moon)
3. Ka-Youbi (火曜日: Tuesday) by Ka-Sei (火星: Mars)
4. Sui-Youbi (水曜日: Wednesday) by Sui-Sei (水星: Mercury)
5. Moku-Youbi (木曜日: Thursday) by Moku-Sei (木星: Jupiter)
6. Kin-Youbi (金曜日: Friday) by Kin-Sei (金星: Venus)
7. Do-Youbi (土曜日: Saturday) by Do-Sei (土星: Saturn)

* In the Japanese language, the sun has another name Ohisama (お日様), in which 日 (Hi) stands for the sun, while the prefix お (O) and the suffix 様 (Sama) are honorifics.

The Kanji characters for the sun and the moon, 日, and 月, have another reading, Nichi, and Getsu, respectively.

(Reference Pages: Wikipedia 七曜, 曜日 )