So, the etymology of Monday was basically explained in entirety yesterday. When Anglo-Saxon invaders crossed into England in the 400s-500s CE, they brought with them their heavily pagan beliefs, including their worship of the Sun and the Moon. The Sun got its day in Sunday, and the moon got its in Monday. A Moon's Day. But that's a bit of an orthographic interpretation; the word moon didn't gain that double o until after the weekday name was formed. In Middle English, Monday was spelled Monenday, and in Old English, it was monandæg. The root here is mona, meaning "moon" and coming from the Proto-Indo-European word for moon, mehns, through Proto-Germanic. Mehns possibly goes further back to meh, meaning "to measure" (a connection between the moon and the cycle it goes through). Day, for the sixth time, was spelled dæg in Old English. This comes from Proto-Germanic dagaz, which in turn derives from Proto-Indo-European deg, with the same meaning.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd