How it is calculated
Under ISO 8601 weeks start on Monday, and week 1 is the week containing the year’s first Thursday (i.e. containing 4 January).
So late-December days can fall in week 1 of the next year, and early-January days in week 52 or 53 of the previous year.
Worked example
21 October 2026 falls in week 43 (Monday 19 to Sunday 25 October) and is day 294 of the year.
Frequently asked questions
How many weeks in a year?
Usually 52; 53 in some years that start or end on a Thursday.
Does numbering differ between countries?
Europe and the Arab world mostly use ISO; the US may count from Sunday.
What is it used for?
Planning, school calendars, production and shipping schedules.