Names of the days of the week

In many languages, the names given to the seven days of the week are derived from the names of the classical planets in Hellenistic astronomy, which were in turn named after contemporary deities, 

The seven-day week was adopted in early Christianity from the Hebrew calendar, who copied it from the Babylonians and gradually replaced the Roman nundinal cycle.  Sunday remained the first day of the week, being considered the day of the sun god Sol Invictus and the Lord's Day, while the Jewish sabbath remained the seventh. Emperor Constantine adopted the seven-day week for official use in 321 CE, making the Day of the Sun (dies Solis) a legal holiday.

The Germanic peoples adapted the system introduced by the Romans by substituting the Germanic deities for the Roman ones (with the exception of Saturday)

In the international standard ISO 8601, Monday is treated as the first day of the week.

https://en.wikipedia.org/wiki/Names_of_the_days_of_the_week