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.