[somat]

Weeks are quarter months. Or are supposed to be. Months have been screwed up for a long time now.

I can imagine in prehistorical times keeping track of midrange time by the phase of the moon, if months were kept synced to the moon we would probably name weeks as such, waxing gibbous, waning crescent...

On the subject of keeping months in sync, I can imagine having a fun festival season around the weird 13th month(all months being 29.5 days long) that bisects the new year.

[sieve]

A lot of Hindu calendars do this. They use śukla and kr̥ṣṇa pakṣa (waxing and waning phases respectively). Days are numbered 0-14 with 0 referring to the full/new moon. Problem is each regional calendar is slightly different. And days might repeat.

[sheept]

I'd've thought modern weeks have biblical origins rather than being based on quarter moon phases.

[somat]

Weeks are largely biblical(with pagan day names, that's fun), but they got it from wanting a quarter month sized unit of time. Not deliberately with design, but I suspect that is how it fell out to satisfy their religious requirements.

The tricky part about months (and weeks by inheritance) is that while the moon phases are a nice easy mid-range timekeeping device(something between days and years) they don't line up with days or years very well. So as your society settles on having 4 important religious observations per month and starts keeping better time. It changes to having that observation every 7 days and delinking the month from the moon so it does not drift around the year so much.

I don't really know how the BCE biblical world did the week keeping. but our modern interpretation was mostly solidified by incorporating approximations of that into the other big influence on Europeans society, the Romans. Who used a month and half month system so it fit fairly well.

[jameshart]

Weeks come first. Electing to choose one of the weekdays and making it a ‘sabbath’ or similar comes next.