A traditional unit of time equal to seven days [3]. The custom of the seven-day week, with one day set aside for rest and religious observance, goes back more than 3000 years to the ancient civilizations of the Middle East. The seven days originally had an astrological significance; there is one day for each of the five visible planets and one each for the sun and the moon. Christians and Moslems inherited the seven-day cycle from the Jewish religion. The Romans picked up the idea from the Persians and were using the week as early as the first century. When the Emperor Constantine legalized Christianity in the Roman Empire, early in the fourth century CE, the Christian version of the week, with Sunday as the day of religious observance, became official throughout the Empire. Since none of the units of Roman date-keeping (the month, the quarter, and the year) equal a whole number of weeks, this made it necessary for the first time to have tables (we call them calendars!) showing the ever-changing relationship between the days of the week and the dates of the month. Link: The World Calendar Association promotes efforts to reform the calendar so that weeks and months would have a fixed relationship. Link: A perpetual calendar provides calendars for any month and year in the current (Gregorian) calendar. Note: There are different traditions as to which day of the week is the first. In the U.S., most calendars show Sunday as the first day of the week, but the International Organization for Standardization (ISO) specifies that the week begins with Monday. There are also different ideas about how to number the weeks of the year, which is sometimes necessary for business purposes. The official solution to this question is that week 1 of the year is the week (beginning with Monday) that contains January 4. By this convention, week 1 of 2010 will be the week January 4-10, 2010. Link: ISO 8601 from Markus Kuhn