Not a great answer for you, but around the world, in all ancient cultures, it made sense to have a calendar as a celestial system, meaning it’s based on planets, stars, the sun and the seasons.
They all figured out that a “year” is a thing, that the seasons and weather repeats, that today of this year is going to be similar, weather wise, to today of next year. So they created the concept of a “year” based on getting the weather right in this sense, for example you should always plant your crops on a certain day every year, or harvest them, etc.
Our calendar gets it’s roots from a specific culture – the ancient Babylonians. While every culture around the world had a similar “year”, they all divided it up a bit differently, concepts of “months” “weeks” etc. varied from place to place.
The Babylonians loved the number 7, especially with mystical things. Why? Because of space. The Moon, Mars, Venus, Sun, Jupiter, Mercury, Saturn are all visible to the naked eye so it made sense in their minds to have 7 days named for each one. *Saturn*day, *Sun*day, *Moon*day, etc.
Now the Babylonian culture was thousands of years ago but this system passed around Europe and North Africa widely. In English our current “words” for the days derive from a mix of Latin and German roots but there are connections.
Jupiter (the planet) is tied to Jupiter (the God). Loosely, the Norse God “Thor” is culturally and thematically linked to Jupiter. *Jupiters*day became *Thors*day became Thursday.
Etc. etc.
Just case you are solely an English speaker, yes, these trends extend to the other Latin languages as well. In Spanish Luna means The Moon. So Moonday = Monday = Lunes.
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