ISO Week Dates Explained

A practical method

ISO 8601 week dates organize a year into Monday-to-Sunday weeks. Week 1 is the week containing the year's first Thursday, which is also the week containing January 4. Because of that rule, the first days of January can belong to the previous ISO week-year and the last days of December can belong to the next one.

1. Use Monday as the first day: ISO weekdays run from Monday 1 through Sunday 7.
2. Find the week containing January 4: That entire Monday-to-Sunday period is ISO week 1.
3. Count complete weeks: Continue numbering weeks until the week containing the target date.
4. Report the ISO week-year: Near New Year, the ISO week-year may differ from the calendar year printed in the date.

Worked example

January 1 can fall on a Friday, Saturday, or Sunday and still belong to the final ISO week of the previous year. A correct result therefore includes both parts, such as 2025-W01 or 2026-W53, rather than showing only a week number.

Checks, edge cases, and common mistakes

• Do not assume January 1 always starts week 1.
• Do not mix Sunday-first commercial calendars with ISO week numbering.
• A year can contain ISO week 53 when the weekday pattern permits it.
• Use the ISO week-year together with the week number in reports and file names.