come straight from Greco-Roman mythology!
To wit:
"The next thing Eurynome did was very interesting, and involves other mythology you might know about. She created the Seven Planetary Powers, putting a Titaness and a Titan over each. Theia and Hyperion were given the Sun and the power of illumination; Phoebe and Atlas were given the Moon and the power of enchantment; Dione and Crius were given the planet Mars and the power of growth; Metis and Coeus were given the planet Mercury and the power of wisdom; Themis and Eurymedon were given the planet Jupiter and the power of law; Tethys and Oceanus were given the planet Venus and the power of love; and Rhea and Cronus took the planet Saturn with the power of peace. If you look at each of the assignments, you'll find they match perfectly."
http://www.paleothea.com/Myths/Eurynome.htmlSo, interpolating from the above, first comes Sun-day, then Mo(o)n-day, then Mars-day, then Mercury-day, then Jupiter-day, then Venus-day, then finally Satur(n)-day. The respective Japanese names are Nichi-yobi, Getsu-yobi, Ka-yobi, Sui-yobi, Moku-yobi, Kin-yobi, and Do-yobi. The Japanese characters used to write the name of the day are the same characters used to denote the corresponding celestial body: Nichi=Sun, Getsu=Moon, Ka=Mars, Sui=Mercury, Moku=Jupiter, Kin=Venus, Do=Saturn.
Pretty trippy.