Origins
Monday, the day of the Moon, stands unique among the English weekdays as the one name that required no mythological substitution when Germanic peoples adopted the Roman planetary week. Its history illuminates the remarkable cultural transfer through which Mediterranean astronomy reshaped the calendar of northern Europe.
The word derives from Old English 'mōnandæg,' a compound of 'mōna' (moon) and 'dæg' (day). This was itself a calque — a loan-translation — of Latin 'lūnae diēs,' the day sacred to Luna, the Roman personification of the Moon. The process by which this translation occurred is known as interpretatio germanica: when Germanic peoples encountered the Roman seven-day planetary week during the first centuries of contact with the Roman Empire, they systematically replaced each Roman deity with the nearest Germanic equivalent. Mars became Tīw, Mercury became Wōden, Jupiter became Þunor, and Venus became Frīg. But the Moon and the Sun, being celestial bodies rather than anthropomorphic deities in Germanic tradition, were simply translated directly.
The Proto-Germanic form is reconstructed as *mēnins dagaz, from *mēnōn (moon), which descends from the Proto-Indo-European root *meh₁n̥s. This PIE root is among the most confidently reconstructed in historical linguistics, with reflexes in nearly every branch of the family: Latin 'mēnsis' (month), Greek 'mḗn' (month, moon), Sanskrit 'mās' (month, moon), Lithuanian 'mėnuo' (moon, month), and Old Church Slavonic 'měsęcĭ' (moon, month). The root is related to PIE *meh₁- (to measure), reflecting the ancient observation that the moon's cycle provided the most natural unit for measuring time — hence the intimate connection between 'moon' and 'month' in English and most other Indo-European languages.
Old English Period
The phonological development from Old English 'mōnandæg' to Modern English 'Monday' involved significant reduction. The middle syllable '-an-' (a genitive inflection meaning 'of the moon') was gradually eroded, and the compound was compressed into two syllables. The vowel in the first syllable shifted from the long /oː/ of 'mōna' to the /ʌ/ of modern 'Monday,' a change reflecting the general shortening of vowels in unstressed or frequently-used words.
What makes Monday linguistically distinctive among the weekdays is its transparency across language families. In the Romance languages, the Latin 'lūnae diēs' produced French 'lundi,' Spanish 'lunes,' Italian 'lunedì,' Portuguese 'segunda-feira' (an exception, meaning 'second fair/day,' reflecting ecclesiastical Portuguese numbering), and Romanian 'luni.' In the Germanic languages, the same celestial reference produced German 'Montag,' Dutch 'maandag,' Swedish 'måndag,' Danish 'mandag,' Norwegian 'mandag,' and Icelandic 'mánudagur.' Because no god-for-god substitution was needed, the two families of languages point to exactly the same referent.
The cultural history of Monday as a day of dread — 'Monday morning,' 'the Monday blues,' the Boomtown Rats' 'I Don't Like Mondays' — is a modern industrial phenomenon with no ancient precedent. In medieval Europe, Monday was actually considered a favorable day in many folk traditions, partly because of the moon's association with silver, fertility, and dreams. The notion of Monday as the grim start of the working week only became widespread with the rise of the factory system in the eighteenth and nineteenth centuries, when the structured workweek replaced the more fluid rhythms of agricultural and artisanal labor.
Legacy
In astrology and medieval cosmology, Monday was governed by the Moon and associated with emotions, intuition, and changeability — qualities still sometimes attributed to people born on Monday in folk traditions. The nursery rhyme 'Monday's child is fair of face' reflects this lunar association with beauty and grace, the moon being connected to feminine beauty across many cultures.