How we became weekly
aeon.co
aeon.co
Friday: ortzirale (L LN), ortzilare (L LN R), orzirale (L), orzilare HN LN R), ostirale (LN), osti(r)ále (Z), ostrále (R), orziláre (R), ostiral (G) n. ‘Friday’. 1622. Final element obscure. See also barikua {~ bariaku under afari.}
Saturday: larunbat (G HN L LN), laurenbat (old B G), laranbate (old LN *) ** n. ‘Saturday’. ** [FHV 95, 491, 501] Many have seen this as laurden ‘quarter’ (see lau [1]) + bat ‘one’, since Saturday completes the week, and a week is roughly one quarter of a month. But M. **{(1971b),} (1977a: 491) cautiously suggests a derivative of lagun ‘companion’, perhaps *lagunen bate ‘gathering of companions’, + -en {[1]} Gen. + a verbal noun from batu ‘get together’ (see bat). Cf. neskanegun (under neska).
Sunday: igande (G HN L LN), igánte (Z R) n. ‘Sunday’. 1545. + -te {[1]} NFS (M. **{1971b: 586 n.12}). Though phonologically perfect, this etymology is semantically odd, and it is not certain, but it seems likely. The original sense was app. ‘ascension (day)’. Cf. Russian voskresén’e ‘Sunday’, originally ‘resurrection’. See domeka.*
These are all from R. L. Trask (2008) "Etymological Dictionary of Basque".
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In Rome, interest and rent were mostly computed by the month, and the days inserted to keep the calendar aligned were deliberately between years to make these calculations easy to do (no interest or rent was due, and they could not be used as start or stop days of contractual relationships).
Using a pencil influenced other modern things too, like the Dow indexes and GDP.
For a similar reason, it might have been better if humanity had settled on a base 12 number system rather than base 10. We can only easily halve 10 while we can easily halve, third, and quarter 12. More factors means it's more intuitive to reason about common fractions of things.
Earlier civilizations did use base 12, and we still have elements of that history in common use, like the special names for eleven and twelve in the English language and our 12 hour clocks. A common theory for the prevalence of myths about 6 fingered foreigners in the Bible is that these other cultures used base 12 numbers, and the only way people using base 10 could understand that was to assume that they must have 6 fingers to count with. Interestingly, you can actually use your fingers to count to 12, using the 3 segments on each of your 4 fingers rather than the 5 whole fingers.
Sure, but the nice 4 week period is not actually anywhere close to a lunar month, so the "groupings of 4 weeks" part makes sense because 4 is a nice number, but the "7 days to a week because 4 of them will add up to a lunar month" thing is just not even close. We're not talking about something like leap seconds where the drift is slow relative to a human lifespan. 28 days is a whole day and a half shorter than a lunar month.
The phases of the moon do have two practical consequences that I can think of: night-time illumination and the height of the tides, but the week/lunar month ratio is far enough from being an integer that these issues vary significantly from week to week. For the same reason, you cannot use the moon to tell where one week ends and another begins; you have to count the days, just as you would for some other choice.
Between lunar phases and seasons, it’s a fairly natural choice for people living outside of the tropics.
Edit: to add, for pre modern people, tracking seasons was obviously very important and worth counting systematically. Simply counting lunar phases isn’t reliable given that a lunar year is only ~354 days.
That is the lunar orbital period, but from earth it appears as though it takes ~29.5 days for the moon to cycle through its phases(this longer period is termed the 'lunation'). The difference is due to the earth-moon system orbiting around the sun, so by the time the moon has completed it's orbital period around the earth, the earth-moon system has rotated around the sun ~1/13 of its period, changing the angle at which the moon is illuminated, which is what determines the apparent phase of the moon as observed from earth. So the moon has to travel a bit further than its full orbit to appear as though its completed a cycle as viewed from earth.
Religions, finally, "officially" decided that a week had to have seven days.
[^1]: https://www.timeanddate.com/calendar/days/7-days-week.html
The Romans had 8-day “weeks” https://en.m.wikipedia.org/wiki/Nundinae