"The origin of the seven-day week is the religious significance that was placed on the seventh day by ancient cultures, including the Babylonian and Jewish civilisations."
"Some Jewish, Christian, and Islamic groups have been historically opposed to the calendar because their tradition of worshiping every seventh day would result in either the day of the week of worship changing from year to year or eight days passing when Year Day or Leap Day occurs.[11] Others have contended that Year Day and Leap Day could be counted as additional days of worship."
"Thus, once or twice a year there would be eight days rather than seven between consecutive Saturdays. Thus the Jewish Sabbath, which must occur every seventh day, would be on a different weekday each year. The same applies to the Christian Sabbath. Hertz realised that this would cause problems for Jews and Christians alike in observing their Sabbaths, and mobilised worldwide religious opposition to defeat the proposal."
The seven-day week, yes, the rest of it is Roman in origin: the solar nature, the months, their names, their order, etc.
One of the weird ones is our (English) weekday names: Sunday, Monday, etc. Those are apparently from German, but after their contact with the Romans, and after the Romans adopted the 7-day week. Which means it is after the Christianization of Rome, but before the Christianization of the Germanic tribes. They took the Roman week and put their gods' names in place instead.
Even more peculiarly, anglo-saxon names actually do not reflect full Christianization of Rome itself: Saturday is still the day of Saturn rather than the Shabbath, and Sunday is still the day of "Triumphant Sun" rather than the "Dominicus" (belonging to the Lord). Which means they originated before the end of IV century AD, when Rome officially introduced these two Christian references in its weekday names (all others were maintained intact).
It has to be said also that Roman calendars were actually "pillaged" from Greek culture and then tweaked until right - a pragmatic approach they followed in so many different cultural and scientific areas when interacting with foreign civilisations. Unfortunately, they didn't go as far as trying to align the 7-day week (of Middle-Eastern origin) with their Greek-ish months, and their legacy ended up being managed by a Church that valued immutability over everything else. So here we are, 1600 years later, still celebrating the months of Julius (Caesar) and Augustus when the weather is the best around the Mediterranean Sea, the month of war-god Mars in the best period to start military campaigns in the same region, and with four months named by numerical positions they've long since changed... and still completely mismatched with 7-day weeks.
