The Babylonians named their days after the 5 celestial bodies they knew, the moon being one of them.
In Anglo-Saxon the name Monday derives from "mondandaeg" which translates in to "the moon's day" .
It is actually the second day of the week in Nordic culture and was reserved for worshipping of their moon goddess.
Another cool source of information Calendar through the Ages
What Is the First Day of the Week?
The Bible clearly makes the Sabbath the last day of the week, but does not share how that corresponds to our 7 day week. Yet through extra-biblical sources it is possible to determine that the Sabbath at the time of Christ corresponds to our current ‘Saturday.’ Therefore it is common Jewish and Christian practice to regard Sunday as the first day of the week (as is also evident from the Portuguese names for the week days). However, the fact that, for example, Russian uses the name "second" for Tuesday, indicates that some nations regard Monday as the first day.
In international standard ISO-8601 the International Organization for Standardization (ISO) has decreed that Monday shall be the first day of the week.
clip from this LINK
RESPECT