Week, a cycle of days used in reckoning time. In the Gregorian calendar a week consists of seven days and begins with Sunday. Unlike the year, the month, and the day, the week is not based on a natural pattern but is an artificial division of time. Many ancient societies had 10-day weeks; so did the French during the French Revolution. The calendar introduced into Rome by Julius Caesar divided each month into three unequal periods.

The seven-day week probably was an outgrowth of astrology. Babylonian astrologers believed that the sun, the moon, and the five known planets each controlled one day in a cycle of seven. The Hebrew custom of reserving every seventh day as a holy day appears to be based on the Babylonian seven-day week. It was adopted and spread through the Western world by the Christians.