The 7-day week originated in ancient Mesopotamia, particularly Babylon, around the 3rd millennium BCE. It was tied to the observation of seven celestial bodies visible to the naked eye: the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Each day was associated with one of these "planets," a system later adopted and refined by other cultures.
The Babylonians passed this concept to the Jews, who formalized it in their religious practices, linking it to the biblical creation story in Genesis, where God created the world in six days and rested on the seventh, the Sabbath. This Jewish tradition influenced the structure of the week in the Western world.
The Romans also adopted the 7-day cycle by the 1st century CE, replacing their earlier 8-day market cycle (nundinae). They named the days after their gods, aligned with the celestial bodies: dies Solis (Sun), dies Lunae (Moon), dies Martis (Mars), dies Mercurii (Mercury), dies Iovis (Jupiter), dies Veneris (Venus), and dies Saturni (Saturn). These names persist in Romance languages (e.g., French: lundi for Moon, mardi for Mars).
The 7-day week spread through the Roman Empire and was cemented by Christianity, especially after Emperor Constantine made Sunday a day of rest in 321 CE. Other cultures, like the ancient Egyptians and early Chinese, had different cycles, but the 7-day week became dominant globally due to Western influence, trade, and colonization.
It’s not perfectly aligned with natural cycles—365.25 days in a year don’t divide evenly by 7—but its cultural and religious roots have made it a near-universal standard.