I had written:
<< But the seven-day week has no basic natural pattern to it. >>
You responded:
<< Except to those who have observed the moon. (Or a woman.) >>
No, that's wrong. Many different cultures came up with "months" by observing the moon. Only one culture came up with the seven-day week. The cycle of the moon is between 29 and 30 days. So -- those cultures that used months had months of 29 or 30 days.
And there really is no logic behind the idea that a seven-day cycle came about from observing a 29-30 day cycle.
Now the woman's period -- you may have something there! I don't mind the idea that the 7-day week may have come from this. What bothers me is that women have an excuse to be crazy for three weeks out of every four!
I knew a woman who used to excuse her bizarre behavior by saying, "I was ovulating!"
The Hindu's had 7-day week. So to Tibet, Burma (although I haven't checked if these were borrowed from the Chinese who borrowed them themselves.) The Egyptians had a 7-day week (at least the followers of the Handsome Sin) which devolved into the Roman week. So the Jews were not the only ones to use 7 days. Sumer didn't use weeks, but they took the first, seveth, and fifteenth of each month off anyway.
Some Egyptians and Greeks (and later French) used a 10-day week. The Aztecs (copying the Mayas) used 13 and 20-day weeks together.
But how is this anything but an historical accident? Other cultures survived, even for millenia without a 7-day week or even without the concept of a week. (Some did do alternating 7 and 8-day weeks.)
Hours, days, weeks, months, eveny years don't come out evenly placed in any calendar construction anyway. Omar Khayyam's was the best for leap years, 31 leap years in 128 years (skip one year evey 128 instead of every 100); tidal drag will change the length of the day relative to the year before the approximation fails (much better than the Julian calendar). The guys working on the Gregorian did know of Omar's work though.
Can one use a julienne-colander for slicing carrots?