ISO Week Numbers: Why Week 1 Is Not Always in January

Week 1 is the week containing the first Thursday

ISO 8601 weeks start on Monday and end on Sunday. Week 1 of a year is the week that contains the first Thursday of January, which is equivalent to saying it is the week containing 4 January, and equivalent again to saying that the majority of its days fall in January. Every week belongs to exactly one week-numbering year.

The week-numbering year is a separate value

Because a week is assigned to the year that contains its Thursday, the week-numbering year and the calendar year disagree for a few days around New Year. Monday 29 December 2025 already belongs to 2026-W01, and Friday 1 January 2027 still belongs to 2026-W53. A week number is ambiguous unless the week-numbering year travels with it.

• Store and display the pair, such as 2026-W01, never the week number alone.
• Expect the week-numbering year to differ from the calendar year in late December and early January.
• Do not derive the week year by taking the calendar year of the date.

Some years contain 53 weeks

A year has 53 ISO weeks when it begins on a Thursday, or when it is a leap year beginning on a Wednesday; otherwise it has 52. Long years occur roughly every five to six years. Reporting code that assumes 52 buckets will silently drop a week, and year-over-year comparisons that align week numbers will drift by a week after a long year.

ISO weeks are not the only week system

A common alternative starts weeks on Sunday and treats the week containing 1 January as week 1, which produces different numbers for the same dates. Spreadsheet functions often default to that convention or offer both. Before comparing two sources of week numbers, confirm that they use the same week start and the same definition of the first week.

Dates where the two years disagree