I just found out that the Japanese names of the days of the week

[Art_from_Ark]

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.html

So, 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.

[Godhumor]

1. A lot of civilizations follow this naming pattern

It is believed to predate the Greco-Roman era maybe as far back as Old Babylonia. More specifically for Japanese days is that they're based on the old Chinese 5 elements (Which had associations with the planets you mentioned).

The wikipedia page for days of the week has a really interesting chart showing the similarities in naming convention for days across cultures.

http://en.wikipedia.org/wiki/Days_of_the_week#Cross-linguistic_overview

Oddly enough, I used this as a lesson in migration when I taught high school social studies. Kids had to fill in a map with naming conventions for each region then try to figure out, based on how the names changed, how it spread. Fun lesson.

[Art_from_Ark]

2. Some modern Japanese calendars also show the Buddhist names

for a 6-day week, that is, (in order from luckiest to unluckiest) Tai-an (supposed to be the best day for weedings), Tomobiki, Sensho, Shakku, Senpu, and Butsumetsu, with the last day being particularly unlucky because it was supposed to be the day of the 6-day week that Buddha died on.

So how did Japan come to adopt a 7-day week based on Greco-Roman mythology?