Some of you may know this already, but I am always in awe when I think about how much ancient Greek culture is still present in our daily lives today — for instance in how we name the days of the week.

In ancient Greece, each day was named for a planet. The week started with the Day of the Sun, followed by the Day of the Moon and the five best visible planets of our solar system.

Greek Translation English
Heméra Helíou Day of Helios / the Sun Sunday
Heméra Selénes Day of Selene / the Moon Monday
Heméra Áreos Day of Ares / Planet Mars Tuesday
Heméra Hermou Day of Hermes / Planet Mercury Wednesday
Heméra Diós Day of Zeus / Planet Jupiter Thursday
Heméra Aphrodítes Day of Aphrodite / Planet Venus Friday
Heméra Krónou Day of Kronos / Planet Saturn Saturday

Helios and Selene are the titan god of the sun and the titan goddess of the moon, respectively.

Ares is associated with the planet we today call Mars, after his Latin counterpart. In ancient Greece, the planet Mars was called "Aster Areos", star of Ares, as Mercury was "Aster Hermou", the star of Hermes and "Aster Diós" the star of Zeus.

Interestingly, the Day of Hermes is the 4th day of the week and the number 4 is sacred to Hermes, as it's said he was born on the 4th day of the month. But I could not confirm this for the other deities. For instance, Aphrodite usually shares the 4th day of a month with Hermes to be honoured and the 6th day is sacred to Artemis instead.

You may wonder at this point how a day like Heméra Aphrodites turned into the English Friday or the French vendredi. Of course what we adopted was the Roman system which was based on the Greek system:

Latin Translation English
Dies Sol Day of Sol / the Sun Sunday
Dies Lunae Day of Luna / the Moon Monday
Dies Martis Day of (the planet) Mars Tuesday
Dies Mercurii Day of (the planet) Mercury Wednesday
Dies Iovis Day of (the planet) Jupiter Thursday
Dies Veneris Day of (the planet) Venus Friday
Dies Saturni Day of (the planet) Saturn Saturday

The system may be even older and the association of the days of the week with the sun, moon, and planets is based on an Egyptian system, according to Cassius Dio, a Roman statesman and historian of Greek origin. From what I could find out, the ancient Egyptians did associate stars with their deities but they had a 10-day-week instead of a 7-day-week and while the names of the 10 stars are known, their relationships to modern star names and constellations remsin a mystery.

So now we can account for the English Sunday, which has remained unchanged in its meaning throughout the millenia, Saturday, which also still denotes the Day of Saturn, and Monday, which is still recognisable as the Day of the Moon.

But what about Tuesday to Friday? They share no similarity with their Latin antetype, unlike the French mardi, mercredi, jeudi and vendredi and other Romance languages. Here, the interpretatio romana comes into play. The people living in the North of the Empire adopted the Roman system to name their days of the week. Only, they used their own deities, unaware that the Romans and Greeks had used the names of their stars originally. So through interpretatio romana we now come to the following:

Old English Translation English
Sunnandæg Day of the Sun Sunday
Mōnandæg Day of the Moon Monday
Tiwesdæg Day of Tiw / Tyr Tuesday
Wōdnesdæg Day of Woden / Odin Wednesday
Þūnresdæg Day of Thunraz / Thor Thursday
Frīġedæġ Day of Frige / Frigg Friday
Sæternesdæg Day of Saturn Saturday

In the case of Saturday, the Roman name was borrowed directly by West Germanic peoples, maybe because none of the Germanic gods were considered to be counterparts of the Roman god Saturn. In Norse and German, other names were used, though: Icelandic laugardagur, Danish & Norwegian lørdag, and Swedish lördag meaning "bathing day" or German Samstag, Sonnabend, meaning "sabbath" or "Sunday Eve", respectively. The meaning of "Sunday Eve" already existed in Old Saxon (Sunnunāƀand) and Old Norse (sunnunótt).

So now you know that even though we are millenia apart, we still use the same days of the week as the ancient Greeks did — adapted to the old gods that once ruled the lives of the peoples of Britain, Scandinavia, and Germany.

Sources