—— Witold Rybczynski
Babylonia: Seven appeared as a magical number among the Babylonians, as early as the third millennium B.C. and played an important role in their calendar. Seven heavenly bodies with apparent motion in the sky: the "erring" seven, the seven "wanderers" that is, the seven planets of antiquity (including the sun and the moon).
→ Greece, Egypt, and Rome
Rome: planetary week (see WEEK | NAME)
Jew: Sabbath, according to the Old Testament it’s their day (on the seventh day of the week—i.e., Saturday.) (adopted this “7” method of timekeeping during their exile in Babylonia)
**ISO week date:** Weeks start with Monday and end on Sunday, from 1 to 7.
For centuries the Romans used a period of eight days in civil practice, but in 321 CE Emperor Constantine established the seven-day week in the Roman calendar and designated Sunday as the first day of the week. Subsequent days bore the names Moon’s-day, Mars’s-day, Mercury’s-day, Jupiter’s-day, Venus’s-day, and Saturn’s-day. Constantine, a convert to Christianity, decreed that Sunday should be a day of rest and worship.