system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some days of the week named after Norse Gods, and some days named after Roman ones?
gpt
There is only one day of the week that is truly of Latin origin: Saturday, named after the Roman god Saturn.

This is because the Germanic peoples took the Roman days of the week and, as best they could, found the equivalent Norse god to name it after -- this matching of Norse gods to Roman gods was called "interpretatio germanico", "the Germanic interpretation".

For example, the Latin dies Martis was named after Mars, the god of war. The Germanic people had a god called Týr (Tiw in English) who was the god of heroic glory and combat -- and so we got Tiwsdæg.

However, there is no Norse god that corresponds to Saturn, so dies Saturni became Sæturnesdæg.

Not all Germanic languages kept Saturn, though. In the Scandinavian languages it is "Lördag" (or similar), which literally means "washing day"; in southern German dialects it's "Samstag" which originally comes from "Sabbath" (even though it doesn't look like it), while in northern German dialects it's "Sonnabend" which literally means "Sunday Eve".