The 7 day week seems natural to people today, but it did not always exist. Its origins lie in the days of ancient Babylon, and it is not the only system that has been tried. The names of the days of the week, although ancient, have been changed over time.
Origins of the 7 Day Week
It is easy to take the 7 day week for granted. After all, its use is near universal today, and after all how else could it really be done? But every system must have a beginning somewhere, and although in use for thousands of years, the 7 day week had to start somewhere.
Not all ancient calendars made use of the 7 day week. The Romans during the Republic, for example, did not have a seven day week, but marked a recurring cycle of 8 days to mark when a market would be held in Rome. The Romans did not have a 7 day week until the Imperial period.
As late as the 18th Century attempts were made to change the calendar. The forces behind the French Revolution understood that the days of the week could be changed. With other changes to the calendar, and even a complete redefinition of our basic units of time (the second, minute and hour), they made a 10 day week. These days were not even given special names, but were called simply primidi (first day), duodi (second day), and so on.
Unsurprisingly this try at calendar reform did not succeed. By this point in time the 7 day system was so ingrained into people that it was difficult to enact any sort of change. Imagine what would happen today if workers were now told that they had a ten day week and weekends would no longer come after five working days. It wouldn’t work.
Our own 7 day week seems to come from the ancient Babylonians over 3000 years ago. Why the 7 day week was chosen is not exactly sure. Some have theorized that 7 was the number of “planets” which could be seen by Babylonian astronomers (Sun, Moon, Mercury, Venus, Mars, Saturn, Jupiter), and this gave significance to the number 7.
It is more commonly believed that the 7 day week was used to approximate the lunar cycle. Like our own calendar, Babylonian months revolved around the lunar cycle, which lasts about 29 and a half days. (Our word month comes from the word moon). Because the lunar cycle did not perfectly coincide with the day, months would be either 29 or 30 days, alternating.
However, the Babylonians also wanted a unit of measurement that would be larger than the day but smaller than the month. 4 weeks of 7 days comes the closest to an even division, if one does not make the weeks last longer than the lunar cycle (five 6 day weeks would close the week cycle after the lunar cycle had already begun again). Because of the significance of 7 with the number of planets, it made sense to have a 7 day week.
The Spreading of the 7 Day Week
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".
Sabado Like Sunday, the name of this day was changed to better fit the Christian week (Sabado comes from the Sabbath).
The days of the week took a different turn in England. In the 1st Century CE England was conquered by the Romans, with great difficulty. (Julius Caesar himself, who had successfully defeated the Gauls in what is now France, attempted to conquer England as well, but was forced back). Being under Roman control the people of England adopted the Roman calendar.
Rome left England in 410, however. With the sack of Rome by the Visigoths, the legions which had been occupying England for the past 3 centuries were more urgently needed in the homeland, and the Romans pulled out.
Left to itself, England continued to follow the 7 day week. However, at the end of the 5th Century and the 6th Century England was invaded, like the Roman Empire itself, by the Germans. In particular the tribes of the Anglos (From which England derives its name) and the Saxons. It is from these two tribes we get the term Anglo-Saxon.
Unlike the Germans who had taken control of what is now France, and Italy, and Spain, the Germans of England did not adopt the Latin language, or the Roman calendar. They did, however, accept the 7 day week, but not without their own changes. The Germans had their own gods, and these would become the basis for the names of the week.
The Germans attempted to find equivalencies between the Roman days of the week and words in their own language. dies solis and dies lunea easily became Sonntag and Montag. Tuesday became named after the German/Norse god Tyw, Wednesday after Woden, Thursday after Thor and Friday after the goddess Frigga. The only day of the week which remained intact was Saturday, for reasons which are not certain, perhaps because they could not find a god in their own mythology to match Saturn, the Roman god of the harvest. This leaves us with the weekdays as we have them now:
The days of the week have remained almost exactly the same since the Anglo-Saxon Conquests, and it can be expected to stay that way. The experiment of the French Revolution demonstrates modern man’s reluctance to move away from the 7 day week. It has become ingrained into the internal clock which we all unconsciously follow. The week has a long tradition, and it will only grow longer with time.