A week is a fixed period of days and although it does not rely on any astronomical basis, it is widely used as a unit of time. The week can be thought of an independent calendar running in parallel with various other calendars. The origin of a seven-week period is generally associated with the ancient Jews and the biblical account of the Creation, according to which God laboured for six days and rested on the seventh.
The ancient Babylonians were known to have observed a fixed seven-day week before the Jews adopted the idea. The Babylonian use of the seven-day week eventually influenced other cultures in Eastern Mediterranian and the Middle East. The use of the fixed 7-day period was probably a simplification of a part of a lunar month. Meanhile both the Babylonians and the Jews retained the lunar while using the 7-day week.
Various groups of citizens of the Roman Empire adopted the week, especially those who had spent time in the eastern parts of the Empire including Egypt where the 7-day week was in use. Contemporaneously, Christians picked up the practice from the Jews and spread the week's use along with their religion.
As the early Christians evolved from being Jewish to being a distinct group various groups evolved from celebrating both the Jewish Sabbath (Saturday) and the first Day or the Lord's Day (Sunday), to only celebrating Sunday.
In the Early 4th Century (CE), the Roman Emperor Constantine regulated the use of the week due to a problem of myriad uses of various days for religious observance, and established the first day as the day for for religious observance for all groups, not just those Christians who were already observing Sunday. The Jews retained their (at least) 800-year-old tradition of Saturday observance. Later, after the establishment of Islam, Friday became that religion's day of observance.
In English the names of the days mostly come from Germanic gods and goddesses: Monday - moon, Tuesday - Tyr, Wednesday - Wodan or Odin, Thursday - Thor, Friday - Freyr, Saturday - Saturn, Sunday - the sun
Days of the week:
Presently, a week is usually considered to begin on a Monday (the weekend being the end of the previous week), though following Constantine's decision to make the first day of the week the day of religious observance, Sunday is considered the first day of the week in some areas. In practice this does not cause any confusion.
The 7-day-week concept defined in the Middle East is now used in most of the world due to the spread of modern commercial trade and bussiness. The word "week" is translated as "star period" in the Chinese, possibly based on some misunderstanding of the translators when the concept was first introduced to China.
Facts and Figures:
1 week = 7 days
1 week = 168 hours = 10,080 minutes = 604,800 seconds
1 year = 52 weeks + 1 day (2 days in a leap year)
In a Gregorian mean year there are exactly 365.2425 days, and thus exactly 52.1775 weeks. There are exactly 20871 weeks in 400 Gregorian years, so 25 December 1601 was a Tuesday just like 25 December 2001.