Use an ISO week together with its week-year
The rule behind the number
ISO weeks run from Monday to Sunday. Week 1 contains January 4, or equivalently the year’s first Thursday. This makes a week a complete seven-day block even when it crosses a calendar-year boundary.
Why January can belong to the previous week-year
The week-year is the year that owns the ISO week, not necessarily the year printed in the date field. Always communicate both numbers when scheduling across New Year: a week number alone can be ambiguous. An ISO week-year contains 52 or 53 weeks.
Other figures on the page
Day of year counts January 1 as day 1. Quarter groups January–March, April–June, July–September and October–December. These are calendar positions; they do not exclude holidays or measure how many days a team is available.
Compare calendar conventions
Some apps use Sunday-start weeks or different first-week rules. If their week number differs, check the calendar convention before assuming either date is wrong. For a sprint or shipment, share the Monday–Sunday date range as well as the ISO label.
Dates near New Year
| Date | ISO label |
|---|---|
| January 1, 2021 | Week 53 of 2020 |
| January 4, 2021 | Week 1 of 2021 |
| December 31, 2026 | Week 53 of 2026 |
Frequently asked questions
Does week 1 always start on January 1?
No. It starts on the Monday of the week containing January 4.
Is the week-year always the calendar year?
No. Dates near New Year can belong to the previous or next ISO week-year.
Does a week number tell me the working-day count?
No. Use a date range and a holiday-aware work calendar for staffing or business-day totals.
See the rules and formulas in our calculation methodology. Found a discrepancy? Send an example for review.
Explanation reviewed October 17, 2026. Example results depend on the stated assumptions.