mōnandæg

The Romans called it dies Lunae, the day of the Moon. Luna was second in the planetary sequence, following Sol. The Roman week assigned each day to a celestial body in a specific order based on Hellenistic astrology: Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus. The Moon drew the second slot.

Unlike most day names, this one survived the Christian reshuffling almost everywhere. French lundi (from lunae dies), Spanish lunes, Italian lunedi, Romanian luni -- all Moon. The Germanic languages translated rather than borrowed: Old English monandaeg, Old High German manetag, Old Norse manadagr. Everyone kept the Moon.

The reason is probably that the Moon had no strong pagan cult to threaten the Church. Mars, Mercury, Jupiter, and Venus were named for gods with active worship traditions. The Moon was just the Moon. The Church fought to rename Sunday (the Lord's Day) and Saturday (the Sabbath) but left Monday alone. It was too harmless to bother with.

In Portuguese, Monday is segunda-feira, 'second fair day' -- part of a numbering system imposed by the Bishop of Braga, Martin, in the 6th century CE. He wanted to strip all pagan names from the week. Portuguese is the only major Romance language where he succeeded completely. Everywhere else, the Moon held its ground.