Back in the day, there was a god for everything. Greeks and Romans had gods for every occasion, including a god for every day of the week. The same rings true for Norse and Viking religion. Obviously, monotheistic religions did not. However, we do already know that religion helped spread the use of certain languages.
Thanks to this multitude of gods we have our days of the week. This is true of most Indo-European languages. Most of these gods corresponded to things witnessed in the sky, namely planets and stars, and from this, we have our week.
We'll start the old-fashioned way, with Sunday.
It doesn't take a genius to work out that Sunday was named after, you've guessed it, the Sun. The biggest and brightest thing in our sky lends its name to what is now part of the weekend and for Christians, a holy day.
The day was initially dies Sōlis in Latin, but for Romance languages such as French, Italian, Portuguese and Spanish, it was changed to the Lord's day, which is dies Dominica in Latin.
In German and English however, it remained the day of the sun: Sonntag in German and Sunday in English.
You can't really honour the Sun without honouring the Moon. From our little world it's the second biggest thing in the sky, and though not responsible for the night, is often semantically related to it since we rarely can see it during the day when the Sun is out. Or in the UK, when the moon isn't.
Mōnandæg was the Old English for it and German still keeps Montag. Romance languages follow the Latin root of dies Lūnae with examples such as lundi (French), lunedì (Italian) and lunes (Spanish). Portuguese by this point has grown weary of the gods and instead just counts the days, segunda-feira being the word for Monday.
In Latin it was dies Martis, and we're talking about Mars, the god of war, which probably should have been reserved for "hump day". The Germanic roots of the day come from the god Týr, the god of law, justice and the sky.
The French have mardi, made famous by mardi gras in English-speaking cultures, especially in New Orleans. The Spanish have martes, the Italians use martedì, and the Portuguese just call it terça-feira, with terça meaning third.
Definitely the hardest day of the week to spell in English. We've even overheard people saying "wed nes day" when writing it out. The day is named after Woden in English, but the Germans opted for something far more efficient by calling it Mittwoch, which literally means midweek.
The Latin is dies Mercuriī, so of course French, Italian and Spanish followed suit and kept their days honouring the god Mercury. Portuguese, however, counts Wednesday as "Day Four".
Named in English after Thor, the god, not the comic book character (though the character is also a god). It's literally Thor's day and in Newcastle-Upon-Tyne, this tradition is still honoured. Just imagine a Geordie saying Thursday...
It's Jupiter who we should be praying to if we're following the Greco-Roman way of thinking. The Latin dies Jovis shaped the words used in most Romance languages, with the obvious exception of Portuguese.
The day isn't named after the ever-fantastic and witty Stephen Fry, though we think it should be. It's named after either the goddess Freya from Norse mythology or Frige, an Anglo-Saxon goddess, though little is known about the latter.
The Romans opted to name their day after a goddess as well. Venus formed the basis of the Latin dies Veneris and, as you can guess, Portuguese just had to be different.
There's a nice little overlap to bring us back full-circle to where we almost started. Saturday in English and the Romance languages honours the god Saturn in the final day of the week... even Portuguese! Clearly Saturn is the best god and planet if everyone can agree on honouring him with the best day of the week. God we love Saturdays!
We also mentioned that Portuguese liked to count the days between Monday and Friday, just like anyone who works in an office. It's not the only language that does this. Ecclesiastical Latin did, which is where Portuguese got the idea. Arabic also does it.