The Global Standard for Week Numbering: ISO-8601
In manufacturing, retail supply chains, corporate budgeting, and software engineering, projects are rarely scheduled by day of the month; they are planned by week numbers (e.g. "Product launch scheduled for Week 42").
Rules of the ISO Week Calendar
- Monday is explicitly defined as the first day of the week (Day 1). Sunday is Day 7.
- The first week of the year (Week 01) is the week that contains January 4th, or the week with the first Thursday.
- Days at the end of December can belong to Week 01 of the subsequent year, and days in early January can belong to Week 52 or 53 of the preceding year.
Frequently Asked Questions
What is an ISO-8601 week number?
Under ISO-8601, weeks begin on Monday and are numbered from 01 to 52 (or 53). Week 01 of any year is defined as the week that contains the first Thursday of that year (or equivalently, the week containing January 4).
Why do some years have 53 weeks?
A regular year consists of 52 weeks plus 1 day (or 2 days in a leap year). These extra days accumulate, resulting in an ISO year with 53 weeks approximately every 5 to 6 years.