The use of the 7 day week proved to be quite popular in the ancient world. The Egyptians quickly picked up on the practice, as did the ancient Hebrews of Israel. It is from their tradition that our own 7 day week originates.
In the 1st Century CE, as Christianity began to grow into its own religion separate from Judaism, they continued with the tradition of the 7 day week, as the celebration their holy day (Sunday or the Lord’s Day) was based upon the 7 day week. (Jesus was resurrected the day after the Sabbath, a Jewish calendar reckoning requiring 7 days in a week).
Although many Romans had adopted the use of the 7 day week, either picking it up from the Egyptians or from the Christians, it did not become standard in the Empire until the time of Constantine. Constantine was the first Roman Emperor to convert to Christianity, and he enacted a 7 day week so that the entire Empire might follow the cycle of the Christian holy day.
Rome was in control of most of Europe at the time of Constantine, as well as much of the Middle East, and use of the 7 day week spread rapidly into those areas where it had not been before. During the barbarian invasions of the 4th and 5th centuries, the conquering Germans decided to keep the 7 day week for themselves.
Naming the Days of the Week
In the beginning, days of the week were named after deities who were seen to be having dominion over a single day. These deities would also correspond to a celestial body. For example, the Latin names for the days of the week were:
Dies Solis Day of the Sun
Dies Lunea Day of the Moon
Dies Martis Day of Mars
Dies Mercuri Day of Mercury
Dies Joves Day of Jupiter
Dies Veneres Day of Venus
Dies Saturni Day of Saturn
Looking at these names it becomes clear that some of these names remain with us. Sunday, Monday and Saturday obviously come directly from these Roman names for the days of the week. In most Latin based languages the names of the week follow even more closely the names used by the ancient Romans. For example look at the Spanish days of the week:
Domingo This is different from the Latin, coming from the Latin Domini or "Lord".
Lunes
Martes
Miercoles
Jueves
Viernes
Sabado Like Sunday, the name of this day was changed to better fit the Christian week (Sabado comes from the Sabbath).