The origins of our days of the week lie with the Romans. The Romans named their days of the week after the planets, which in turn were named after the Roman gods:
- dies Solis “the day of the sun (then considered a planet)”
- dies Lunae “the day of the moon”
- dies Martis, “the day of Mars”
- dies Mercurii, “the day of Mercury”
- dies Iovis, “the day of Jupiter”
- dies Veneris, “the day of Venus”
- dies Saturni, “the day of Saturn”
When the Germanic-speaking peoples of western Europe adopted the seven-day week, which was probably in the early centuries of the Christian era, they named their days after those of their own gods who were closest in attributes and character to the Roman deities.
It was one of those peoples, the Anglo-Saxons, that brought their gods and language (what would become English) to the British Isles during the fifth and sixth centuries AD.Wikimedia Commons
In English, Saturday, Sunday and Monday are named for Saturn, the sun and moon respectively, following the Latin.
The remaining four days (Tuesday, Wednesday, Thursday and Friday) are named for gods that the Anglo-Saxons probably worshipped before they migrated to England and during the short time before they converted to Christianity after that.
Tuesday is named for the god Tiw, about whom relatively little is known. Tiw was probably associated with warfare, just like the Roman god Mars.
Wednesday is named for the god Woden, who is paralleled with the Roman god Mercury, probably because both gods shared attributes of eloquence, the ability to travel, and the guardianship of the dead.
Thursday is Thunor’s day, or, to give the word its Old English form, Thunresdæg “the day of Thunder”. This sits beside the Latin dies Iovis, the day of Jove or Jupiter. Both of these gods are associated with thunder in their respective mythologies.
You may recognise a similarity here with the name of the famous Norse god Thor. This may be more than coincidence. Vikings arrived in England in the ninth century, bringing their own very similar gods with them. Anglo-Saxons were already Christian by this time, but may have recognised the similarity between the name of their ancestors’ deity Thunor and the Norse god. We don’t know, but the word Thor does appear in written texts from the period.IMDB
Friday is the only weekday named for a female deity, Frig, who is hardly mentioned anywhere else in early English. The name does appear, however, as a common noun meaning “love, affection” in poetry. That is why Frig was chosen to pair with the Roman deity Venus, who was likewise associated with love and sex, and was commemorated in the Latin name for Friday.
The concept of the week, that is, a cycle of seven numbered or named days with one of them (usually Sunday or Monday) fixed as the first, was originally probably associated with the Jewish calendar. This was complicated by the fact that early medieval Europe inherited its idea of the week from imperial Rome, via the Christian church.
In early Christianity the reckoning of time was crucial to the proper celebration of the church’s feast days and holidays, especially the variable feast of Easter.
We find day names similar to English in related European languages, like Dutch, German, and all the Scandinavian or Norse languages. Gods with comparable names, like Tyr, Othinn, Thor and Frigg, were certainly known to the Scandinavians and gave their names to weekdays in Scandinavian languages (compare Modern Danish tisdag, onsdag, torsdag, fredag).
The Latin names for the days of the week, and the Roman gods for which they were named, still live on in all the European Romance languages, like French, Spanish and Italian. Think of French lundi, mardi, mercredi, jeudi and vendredi, for example, and you will find the Latin Luna, Mars, Mercurius, Iovis and Venus hidden behind them.