When I told my daughter Naomi about the “beautiful example” of the Kuzari which I always accepted as “this must be true”, she objected, and what can I answer her now?

From the Kuzari:

However, the rabbi stresses that all of humanity has a common origin, as evidenced by the universally accepted seven-day week and decimal system of numbers.

HaLevi, Rabbi Yehudah. The Kuzari: Arguments in Defense of Judaism (Kindle Locations 345-346). Chanan Morrison. Kindle Edition.

From the Internet :

Min Yan, Knows China and experienced a lot of the world
Answered April 30, 2018
The globalisation already standardised the 7 day week in most part of the world. Historically, the weeks are synchronised with local trade. People usually go to local market once every certain number of days, and this becomes the cycle of people’s life, which is effectively the week for these people. For example, Chinese divides each month into 3 segments called 旬Xun (upper, middle and lower Xuns), each lasting 10 days. One can see that the Chinese division is actually much more natural because it is coherent with the natural cycle. A traditional Chinese week can be 10 days or 5 days (if more frequent trades are needed).

The 7-day week is not universal.

However, there is an objective reason for choosing seven days for a “week” – this length of time is equivalent to a quarter of a lunar cycle. It’s the time from a new moon to a waxing half-moon, from a waxing half-moon to a full moon, from a full moon to a waning half-moon, from a waning half-moon to a a new moon. The ancient Babylonians counted these 7-day periods in their calendar. The ancient Gauls counted the 14-day period from new moon to full moon in their calendar.

That said, other cultures have used, and still do use, other week lengths:

  • The ancient Romans used an 8-day cycle, with market day being held on every 8th day. The cycle from market day to market day was called a “nundinum”, from “nine days” (they counted the first market day and the second market day in their total).
  • The Javanese people use a 5-day cycle called a pasaran.
  • The Aztecs used a 13-day week.
  • The ancient Chinese used a 10-day week.