The word Monday comes from ‘Mani’, the old German moon god, which became ‘Mona’ in old English.
Since Sunday is considered the day of the sun, the following day is considered the day of the moon, and this is not only true in English, but in many other languages too, such as Japanese and Sanskrit.
In case you have forgotten what it looks like, here is a picture of the moon.
thiNK TWice 
It looks much the same as this side!
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In French, it is more straightforward, as “lundi” is “day” (“di”) of the “moon” (“lun” or “lune”).
Would like to see the hidden side of the Moon!
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