Why there are seven days in a week?

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 civilizations. Babylonians celebrated a holy day every seven days, starting from the new moon, then the first visible crescent of the Moon, but adjusted the number of days of the final "week" in each month so that months would continue to commence on the new moon. (The seven-day week is only 23.7% of a lunation, so a continuous cycle of seven-day weeks rapidly loses synchronization with the lunation.) Jews celebrated every seventh day, within a continuous cycle of seven-day weeks, as a holy day of rest from their work, in remembrance of Creation week. The Zoroastrian calendar follows the Babylonian in relating the seventh and other days of the month to Ahura Mazda.^[1] The earliest ancient sources record a seven-day week in ancient Babylon prior to 600 BC. ^[2]

The seven-day week being approximately a quarter of a lunation has been proposed (e.g. by Friedrich Delitzsch) as the implicit, astronomical origin of the seven-day week. Problems with the proposal include lack of synchronization, variation in individual lunar phase lengths, and incompatibility with the duodecimal (base-12) and sexagesimal (base-60) numeral systems, historically the primary bases of other chronological and calendar units. For instance, the Chinese Han Dynasty (from 206 BCE) used five-day and ten-day cycles. There are no historical Jewish or Babylonian records that confirm that these cultures explicitly defined the seven-day week as a quarter of a lunation.