A week is a unit of time equal to seven days. It is the standard time period used for short cycles of days in most parts of the world. The days are often used to indicate common work days and rest days, as well as days of worship. Weeks are often mapped against yearly calendars. There are just over 52 weeks in a year. The term "week" may also be used to refer to a sub-section of the week, such as the workweek and weekend.
Ancient cultures had different "week" lengths, including ten days in Egypt and an eight-day week for Etruscans. From Wikipedia
