Readers reply: why does a week have seven days?

Why does a week have seven days? What would happen if we had, say, an eight-day week or a six-day week? Kevin Dawson, London

Send new questions to nq@theguardian.com .

Readers reply

An eight-day week is not enough to show I care . EddieChorepost

As far as I’m concerned, we can dispense with all the other days and keep Saturday. Nexus41

Because the moon’s cycle is 29.5 days and in four cycles: first quarter, full moon, last quarter, new moon. So, astronomically speaking, a seven-day week works. yogainspain

Has a five-day week been tried anywhere? It has the advantage that five is a factor of 365, so there would be more alignment between the days of the week and the dates of the year than we have now. And there would be more time off: three days on, two days off, for 73 weeks. What’s not to like? RebeccaNestor

I believe it was originally a Greek or Babylonian idea, using the five visible planets and the sun and the moon. Just imagine: if we didn’t have seven days, Craig David would never have burst on to our music scene … James

The seven-day week is a social construct that has persisted for millennia due to its practicality. Changing it would challenge cultural norms. The story of creation in Judaism lends weight to the idea of a seven-day cycle, with God resting on the seventh day, but the Babylonians established the seven-day week in about 2300BC. A shorter or longer week would throw months and years out of sync and disrupt established traditions such as the weekend. Micky Gee

I suspect because six days on the trot, even as hunter gatherers, is about as much as most of us can take without needing some kind of break. Tedami7

The Mayans used a cycle of 13 days . Socialismnow

Related: Mayan calendar: much ado about zero | Alex Bellos

Years (Earth circuits of the sun) are a natural multiple of days (rotations of the Earth), but 365 (or 366) days is too many to easily track if you’re dependent on being in position for, say, hunting migrating animals or gathering wild fruits. Moon phases are an easily observed way of grouping the passing of days into manageable months. Structures erected in the mesolithic period , 10,000 years ago, are thought to have been used for tracking lunar phases.

Marking shorter periods, in turn, arises from dividing the lunar month into half, and half again, to give seven nights. The seven-day week was established as a practical division of moon phases by the Babylonians, and the number left traces in the language from where we get fortnight (14 nights) and sennight (an archaic word for a week). From the Babylonians, the reckoning passed to the Assyrians, the Jews, the Persians, the Alexandrian Greeks and others in the Middle East. Seven days wasn’t the only possibility; the Egyptians worked with days in groups of 10, but the Babylonians had the handier way of recording and propagating their system on cuneiform tablets.

Independent of the conventions to the east of the Mediterranean, the pre-imperial Roman calendar was based on days from the start or the end of the month, or days to or from the midpoint, as in “beware the Ides of March”, or however many days after the Ides. As the technology and organisation levels of the Roman empire developed, there was a clear advantage in the practicality of fixed blocks of time. Adopted by the Romans, spread throughout their empire and continued by the Latin church, the Babylonian numerical conventions turned out to be the ones that lasted into the modern age. Their seven-day week prevailed, along with their 60 minutes in the hour and 360 degrees in a circle. leadballoon