My last revelation, when I was in Italy, was that Monday means "Moon day" (guess that makes sense, after the Sun-Day).
Realized that because it's Lunedi in Italian. Never thought about this before.
Yep, as Takh noted, the Germans just copied the Roman calendar wholesale:
Dies Lunae - Day of the Moon - Monday
Dies Martis - Day of Mars - Tuesday [Tiu/Tyr - Germanic war god]
Dies Mercurii - Day of Mercury - Wednesday [Woden/Odin - Germanic god of commerce and travelers]
Dies Iovis - Day of Jupiter - Thursday [Þunor/Thor - Germanic thunder/storm god]
Dies Veneris - Day of Venus - Friday [Frigg/Freyja - Germanic god of beauty and lust]
Dies Saturni - Day of Saturn - Saturday (direct translation)
Dies Solis - Day of the Sun - Sunday
A lot of Romance languages adopted from Latin an alternative word for Sunday, usually referencing the fact that Sunday was the Lord's Day (Dies Dominica - domingo, dimanche, domenico, etc.), and likewise the fact that Saturday was the historical sabbath - sábado, samedi, sabato, etc.
I don't speak Greek, someone else can do those equivalents.