The gods in the English days of the week
English speakers say the names of old Germanic gods several times a week without thinking about it. Tuesday is Tiw’s day, Wednesday Woden’s, Thursday Thunor’s, Friday Frig’s. In Old English they were Tīwesdæg, Wōdnesdæg, Þunresdæg and Frīgedæg, next to Sunnandæg and Mōnandæg for the sun and moon.
These are the same gods the Norse called Týr, Odin, Thor and Frigg. But the names do not record what anyone worshipped on a given day.
A translated week
The seven-day week with a planet for each day is Roman. Each day belonged to a planetary god: Mars, Mercury, Jupiter, Venus and Saturn, with the sun and moon. When Germanic speakers took over the week, they put their own gods in place of the Roman ones. Mars became Tiw, Mercury became Woden, Jupiter became Thunor, Venus became Frig. The sun and moon simply kept their days, which is why Sunday and Monday need no god to explain them. Scholars call this interpretatio germanica.
When exactly it happened is uncertain. The usual estimate is some time after about AD 100 and before the conversion to Christianity in the 6th and 7th centuries. It happened among the West Germanic peoples, the ancestors of the English, Germans and Dutch; the Scandinavians borrowed the names from them rather than translating from Latin themselves.
Saturday and Friday
One Roman god stayed in English: Saturday is Sæternesdæg, Saturn’s day. The Norse did something else. Their Saturday was laugardagr, the washing day, from laug, a bath, with no god in it at all.
Friday has a small puzzle. Old English Frīgedæg clearly means Frig’s day. The Old Norse frjádagr could be Frigg’s or Freyja’s; Freyja’s day is possible, but not certain.
Iceland changes the names
In Old Norse the week ran sunnudagr, mánadagr, týsdagr, óðinsdagr, þórsdagr, frjádagr, laugardagr. Icelanders today say something quite different. Bishop Jón Ögmundsson of Hólar, who died in 1121, is credited with removing the gods: Tuesday became þriðjudagur, the third day, Wednesday miðvikudagur, midweek day, Thursday fimmtudagur, the fifth day, and Friday föstudagur, the fast day. Icelandic still uses them. So the language closest to Old Norse has lost the Norse gods from its week, while English, which never had Norse names for its days, has kept the Old English ones.