As we all know, the ancient Greeks and Romans had a 7-day week, where they named the days Moonday, Marsday, Mercuryday, Jupiterday, Venusday, Saturnday, and Sunday. The Romance languages mostly preserve this (except they call Sunday Lordsday, dies Dominica), but turns out that the Germanic languages have the exact same paradigm (except for Saturday): Mániday, Týrsday, Óðinsday, Thorsday, Frīgsday, Washingday, Sonnaday. Máni is the personification of the Moon, Týr a god of combat (like Mars), Odin/Woden a psychopomp (like Mercury), Thor the god of thunder (like Jupiter), Frīg as Venus was known as Frīg's star, and the Sun is, well, the Sun. Though note that English has Saturday in a closer Romance loan, rather than a descendant of laugardagur (launderday?).

Anyway, that's neat, and you probably already knew that. However! I encountered the Japanese 月曜日, and saw the 月 and thought "Huh, Monday is Moonday in Japanese as well? What a coincidence."

Not a coincidence at all, actually! Turns out the Chinese discovered the planet-named 7-day week in the 4th century AD, whence it was transmitted to Japan before 1000 AD and used for astrological purposes before being promoted to the official week naming system as part of Westernization. Thus modern Japanese has 月曜日, 火曜日, 水曜日, 木曜日, 金曜日, 土曜日, 日曜日, where 月 and 日 are the Moon and the Sun, and the days between named for the planets, each of which is associated with a Chinese classical element: Mars with fire 火, Mercury with water 水, Jupiter with wood 木, Venus with gold 金, and Saturn with earth 土. Chinese has replaced them with numbered days, Week 1 Week 2 etc (though Sunday is still 周日), but in Japanese, they remain. So if you're ever going WTF at the Japanese (or Korean!) names of the week, just blame the Ancient Greeks!