And, according to the following page, the planetary day names eventually made their way to Japan in the first millenium, by way of India and China:

http://www.cjvlang.com/Dow/dowjpn.html

I was always curious about the Friday -> "gold" day -> payday link, but I guess this was a total coincidence. The actual derivation seems to be Friday <- Venus's day -> gold planet's day.

Other parts of the same site are devoted to the European, Chinese, and Vietnamese day name origins:

http://www.cjvlang.com/Dow/index.html#TOP