A week is a period of time comprising seven days. There are 52 weeks in one calendar year. In the Bible, God took six days to create the earth and all the creatures thereon, and then rested on the seventh day. This is the basis for our modern week. In Judaism the seventh day or sabbath is equivalent to Saturday. Sunday is the first day of the week and Saturday is the last. Christians consider Sunday to be the day of rest while in Islam it falls on Friday. The names of most of the days of the week are based on ancient pagan gods. For this reason groups like Quakers will usually not refer to them by name, preferring "first day", "second day", etc.
In western society a working week is usually considered to run from Monday through Friday with Saturday and Sunday constituting the weekend. However, many individuals are obliged to work at the weekend and take other days in lieu.