score:6
It's because of the moon. Mostly.
The new moon was the start of a new lunar month (the word itself being derived from words for moon). A lunar month is 29.53 days and is a very prominent cycle in all early cultures.
7 days is, very roughly 1/4 of a lunar cycle or "lunation" (more like .23 than .25 as you can see from the length of the lunar month). And yes, 7 days does work out nicely with some religious beliefs. But, unfortunately, the Moon's orbit is complicated and doesn't neatly divide as it's independent of the rising and setting sun. So then you fudge the number of days at the end of a month when the next new moon arrives too early or you add a day here and there to make it work out overall (which is why our months are not all the same).
Why 12 months? Well, the four seasons start over after 12 months.
Or, a gΔar is twelfe mΕnath, and seofon daeg make a mΕnath, m ra and m re. :)
Upvote:10
I'm afraid the answer is that we don't know.
The Romans gradually replaced their 8 day week (the imperial nundinal cycle) with a 7 day week over a course of a century, after Julius Caesar's calendar reform in 46 BC. Their reasons for doing so are unclear, however we do know that the two cycles co-existed for quite some time. Ultimately, the nundinal cycle fell in disuse and the 7 day cycle prevailed.
One interesting hypothesis, supported by the names of the days in the Greco-Roman world, is that the 7 day cycle prevailed over the nundinal cycle because of its astrological symbolism: Each day represents one of the seven classical planets. While we don't know for sure what prompted the switch, Hellenistic astrology seems like a far more likely influence for the Romans than Jewish culture.
In 321 AD, the 7 day week - by then the norm - was officially adopted by Emperor Constantine. Constantine's primary reason for normalizing the length of the week seems to have been for all his subjects, regardless of religion, to observe the day of the sun.