The idea of the 7 day week was introduced to Germanic peoples by the Romans, and so Germanic speakers mostly just replaced the Latin names of these days with Germanic gods or words that seemed like the best translations of the Roman words.
The 7 names the Romans gave to their weekdays coincided with the names they gave to the 7 “classical planets” they could see moving through their skies (including the Sun and Moon).
Latin dies Lunae (day of the Moon) was translated to Proto-Germanic “Meniniz Dagaz” (day of the moon), or possibly later into Proto-West-Germanic as *mānini dag. This ultimately evolved to Enlgish “Monday”.
Latin diēs Mārtis (day of Mars, god of war) was translated to a name celebrating the Germanic god of war, who was known as Tyr to the Norse, and Tiw to the Old English. This gave us “Tuesday”.
Diēs Mercuriī (day of Mercury, messenger god) was translated to the day of the god Wodanaz, better known by his Norse name “Odin”. To the English he was Woden, which intimately gave us the oddly spelled “Wednesday”
Diēs Iovis (day of Jupiter, thunder-god) was translated to be the day of the Germanic thunder-god Thunraz, best known by his Norse name “Thor”. The English had their own version of this god, Thunor. The word “Thursday” either comes from the Old English name for this day, or was possibly borrowed from the Norse name (þórsdagr, after Thor).
Diēs Veneris (day of the love goddess Venus), was translated to be the day of Frijjo, Germanic goddess of love and marriage. To the Norse she was Frigg, and to the English, Frig. Her day is Friday.
Diēs Sāturnī (day of the god Saturn) is unique in having been directly plugged in some Germanic languages, rather than being translated, giving us Saturday after Saturn.
And finally diēs Sōlis, day of the Sun, is translated to eventually give us Sunday.
It’s amazing how much history is behind even these simple words we use daily. Each one is the relic of a complex interplay between ancient languages, cultures, and pagan religions from thousands of years ago.
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