So I've known for a long time that Tuesday, Wednesday, Thursday, and Friday were named after Norse gods but often wondered why Saturday was named after a Roman god instead. I've also wondered, ever since I took Spanish I back in high school, how other languages' daynames originated. Last night I found out.
The earliest form of the modern days of the week seem to have originated with the Greeks, and (if I understand this correctly) spread to the rest of Europe by Greek Orthodox missionaries in the early Christian era. Their daynames were astrological--named after the sun, moon, and known planets. These were more or less accepted (in Latin) by the Romance-language-speaking regions of Europe, but changed in the Teutonic regions. Specifically, since the planets were named after Greco-Roman gods, the daynames were altered to reflect the corresponding Norse gods. Thus:
(Still Sun Day in English, but changed to Lord's Day in Spanish and French.)
(Still Moon Day, effectively, in all three.)
Mars is a war god. Tyr is a Norse war god (one of several, but presumably the most like Mars). Hence Tyr's Day --> Tuesday.
I was a bit confused by this one at first. Mercury was very much a messenger/trickster/traveller/thief god, and doesn't seem to link well with Odin, a god of all things but primarily war, leadership, and honor. Then I remembered that Mercury and Odin are both associated with wisdom; in fact, they are both said to have invented writing. That's enough of a link. Odin's Day --> Wednesday.
I didn't figure this one out until I remembered that Jupiter (the god) was also known as Jove; hence Jueves and Jeudi. This one's pretty clear; Jupiter was a thunder god and so's Thor. Thor's Day --> Thursday. Interesting note--Hazel tells me the German word for Thursday literally translates to Thunder Day. (She also tells me, though, that the German word for Wednesday literally translates to Mid-Week, and that the German word for tank literally translates to Armored Cupboard.)
Venus, aka Veneris, is a fertility goddess; the closest Norse goddess would be Freya. Hence Freya's Day --> Friday.
This is the weird one. I assume Spanish and French replaced it with their equivalent of "sabbath" somewhere along the line, or it could be a corruption of Saturn. Apparently our forefathers couldn't think of a Norse god that would correspond to Saturn, a god of time and aging. Hm. Hel or Elli, perhaps?
Anyway, I had a lot of fun thinking through this one. I wish someone had told me about this back in my high school Spanish class; knowing the mythological link would've made it so much easier to remember the days of the week!