Week Number

The ISO 8601 standard defines the week numbering used internationally, known as the ISO week date. Each week starts on Monday and ends on Sunday. Week 1 is the week containing the first Thursday of the year.

Because of this rule, dates near the start or end of a calendar year may belong to a different ISO year. For example, January 1st might fall in week 52 or 53 of the previous year, and late December dates can belong to week 1 of the next year.

Week numbers are commonly used in business planning, project management, and scheduling across Europe and many international organizations.