The idea of the week, as a subdivision of the month, seems to have arisen in Babylonia, and spread westward through Aramaea and Canaan, where each lunar month was divided into four parts, corresponding to the four phases of the moon. The first week of each month began with the new moon, so that, as the lunar month was one or two days more than four periods of seven days, these additional days were not reckoned at all. Every seventh day (sabbatum) was regarded as an unlucky day.
A week of seven days is proclaimed in the first chapter of the Torah of the Israelites. With the development of the importance of the Sabbath as a day of sanctification and emphasis laid upon the significant number seven, the week became more and more divorced from its lunar connection, so that by the time of the second Temple it was merely a period of seven days and no longer depended on the new moon. The new moon, heralding a new calendar month, is observed by the Israelites as the holiday of Rosh Chodesh according to the lunar cycle, while the days of the month are calculated according to the rules of the lunisolar Hebrew calendar. From Judaism the week passed over to Christianity, and through the influence of the latter was generally adopted throughout the Roman empire.
The Genesis account of the creation of the world in 6 days, the 7th day being a rest day, is a mythical attempt to account for the institution of the Sabbath.
