Religion and seasonality. 7-days as a unit of measurement is written into unamendable documents hold as holy by all Judaist monotheisms (Jewish, Christian and Islamic). The ciclical experience of seasonal-weather and daylight-lenght changes are also too powerful to deny.

Unrelated, why do you call Islam "judaist"?

AFAIK, Islam traces its roots back to Abraham, who was Judah's great-grandfather. So it would seem more appropriate to call it an Abrahamic religion, no?

I'm not a religious scholar by any means (and certainly less versed in Islamic tradition than any Muslim), so you're probably correct.

Many people live in places where summer and winter are different, and they happen at the period of a year.