A week is a measurement of time that is equal to 7 days in the Gregorian calendar, which is the standard calendar used in much of the world. The term "week" can be used to describe any period of 7 days, and does not necessarily have to refer to a calendar week. Below are some relationships between different measurements of time within the Gregorian calendar, the most widely used calendar standard, that originated in 1582.
1 week = 7 days
~4.348* weeks = 1 month
~52.178* weeks = 1 year
*the asterisk indicates that these are estimations.
Days in a week
On top of being a period of time equal to 7 days, week is also used to describe a specific cycle of days. The international standard (ISO 8601) names the first day of the week Monday. The rest are as follows:
In some regions, different standards are used. For example, in the United States, the first day of the week is Sunday, but the rest of the days follow in the same order, and repeat from the first day once the last day is reached. There is no real concensus, so what is defined as the "first day of the week" varies depending on who you ask, but is usually limited to Saturday, Sunday, or Monday.
"Week" is also often used as part of a term used to describe periods of time that may involve differing numbers of days, or specific days. For example, a workweek or school week is made up of the days that those specific activities occur, usually 5 days of the full week. Similarly, the week is often broken up into weekdays and the weekend. Weekdays are when work and school usually occur. The weekend are considered rest days. There are 2 rest days per week in most countries. Which specific days make this up can depend on where you live, similar to how what is considered the first day of the week can differ. In some regions, the weekend occurs on Friday and Saturday. In others, the weekend occurs on Saturday and Sunday. These are the two most common definitions of the weekday and weekend, but others exist. In addition, many professionals can also have rest days outside the weekend. For example, in a country with Saturday and Sunday defined to be weekend, a waiter in a restaurant or a nurse in a hospital may work in the weekend and rest on Tuesday and Wednesday instead.