The days of the week were named after the seven classical planets of astrology (by the Romans, who in turn named the seven classical planets, and hence the days, after Roman deities).
The names were taken up by speakers of Germanic languages, and eventually underwent semantic loan translation: Germanic deities were substituted for the Roman ones (with the exception of Saturday). Some people say that the Germanic deities themselves were culturally identified as one and the same as the Roman deities, but we probably won't ever know for sure (interpretatio germanica, analogously to the interpretatio graeca, the identification of Egyptian, Roman and Greek deities [1]).
And now, a minor correction: The day of Venus, Friday bears the name of the Germanic goddess Frigg, who is not the same as the Norse goddess Freyja. The meeting of the two characters, in a debate against Loki, is described in the Poetic Edda [2].
[1] https://en.wikipedia.org/wiki/Interpretatio_germanica [2] https://en.wikipedia.org/wiki/Lokasenna