We have already discussed the origin of the names of Saturday, samedi, and Sunday, dimanche, and these are in fact the most complex of the seven day names. The remaining seven are relatively simple, being named after gods or the planets that represent those gods.

In all of the languages previously cited, the day following Sunday is dedicated to the moon. As a reminder, here is a chart of the names of Monday in all those languages:

The Romans named this day after the moon, whose name in Latin is luna. This is a feminine noun and the genitvie (possessive) form is therefore lunae, giving dies lunae or lunae dies, “day of the moon”, as the day name in Latin. (The Babylonian Sin is also feminine.)

In Vulgar Latin, the Latin spoken by ordinary people in France, this “correct” Latin phrase mutated into lunis dies, perhaps because the genitive forms of the names of the other gods/planets also ended in -is, for example martis dies, the day named after the god/planet Mars.

As time passed, the speech of the people gradually evolved into new forms and the exact syntax of the day names ceased to be important. Thus lunis dies changed into various forms such as lunedi and the modern lundi.

The Germanic and Scandinavian peoples frollowed the Roman pattern of day names except that they replaced the Roman gods with their own. The moon, of course, was common to all of these cultures. In Anglo-Saxon, the word for moon has two forms, mona and mone which are masculine and feminine, respectively. However, their genitive forms coincide as monan. Thus this day was known to the Anglo-Saxons as Monandæg which happens to be a word-for-word translation of the Latin lunae dies.

In Middle English, the word became Monedai (‘g’ in Anglo-Saxon was often pronounced like a modern ‘y’) and this eventually mutated into the modern Monday whose meaning “moon day” is still easy to see.