Week number to date

ISO 8601 defines the week, and its two rules explain everything that looks wrong about week numbers: a week runs Monday to Sunday, and week 1 is the week containing the first Thursday of January — equivalently, the first week with four or more days in the new year. So week 1 of 2026 begins on Monday 29 December 2025, and the last days of December 2025 belong to week 1 of 2026.

That is why the ISO week-numbering year is a separate thing from the calendar year, and why reporting that groups by week and by calendar year double-counts or drops a few days every year. If your reporting is weekly, the year to group by is the week-numbering year.

Most years have 52 weeks and some have 53. A year has 53 exactly when it starts on a Thursday, or starts on a Wednesday and is a leap year. 2026 starts on a Thursday, so it has 53. Asking for week 53 of a year that has 52 gets you a note rather than a wrong date.

A warning about other systems: US convention often numbers weeks Sunday-to-Saturday with week 1 being whichever week contains 1 January, which gives different numbers from ISO for most of the year. Excel’s WEEKNUM defaults to that convention; ISOWEEKNUM is the one that matches this.