Old English 'monandaeg' — moon's day, a loan-translation of Latin 'lunae dies.'
The second day of the week in many cultures, following Sunday and preceding Tuesday.
From Old English 'mōnandæg,' meaning 'day of the moon,' a calque of Latin 'lūnae diēs' (day of the Moon). The Germanic peoples adopted the Roman system of naming weekdays after celestial bodies and their associated deities, translating each Roman name into the nearest Germanic equivalent. Since the Moon had no distinct deity in the Germanic pantheon, the celestial body's name was retained
Monday is the only English weekday whose name translates perfectly across both Germanic and Romance languages — German 'Montag,' French 'lundi,' Spanish 'lunes,' and Italian 'lunedì' all mean 'moon day,' because no Germanic god was substituted for the Moon.