The seven-day week is one of the most universal and durable human inventions, yet most of us take it for granted. Unlike the day, month, or year, the week has no direct, obvious astronomical basis. So why did this seven-day rhythm conquer the calendar and become the global standard?

The Astronomical Basis: The Seven Visible "Planets"

The earliest known source for a seven-day cycle comes from ancient Mesopotamia. The Babylonians and Sumerians observed seven celestial bodies that appeared to move against the fixed stars: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. They considered these seven "planets" to be divine and began to associate each one with a day, creating a seven-day cycle.

The Jewish Tradition and the Sabbath Cycle

The most influential force in establishing the seven-day week was the Jewish tradition. The Book of Genesis describes a seven-day creation story, culminating in a day of rest—the Sabbath. This established a religious and cultural cycle that was not tied to celestial events but to divine law. The continuous, unbroken rhythm of the Sabbath cycle became a cornerstone of Judaism and, later, Christianity.

The Roman Adoption and the Naming of the Days

The Romans initially used an eight-day market cycle. However, as the Empire expanded and came into contact with Eastern cultures, the seven-day week grew in popularity. Emperor Constantine officially adopted the seven-day week for the Roman Empire in 321 CE. The English names for the days are a fascinating mix of Roman and Norse traditions, linking each day to a celestial body or a corresponding god:

• Sunday: Sun's Day
• Monday: Moon's Day
• Tuesday: Tiw's (the Norse god of war, Mars) Day
• Wednesday: Woden's (the Norse god Odin, Mercury) Day
• Thursday: Thor's (the Norse god of thunder, Jupiter) Day
• Friday: Frigg's (the Norse goddess of love, Venus) Day
• Saturday: Saturn's Day