Why does the "week-numbering year" sometimes differ from the calendar year?

Near a year boundary, a date can belong to the adjacent year’s week — e.g. December 31, 2018 falls in week 1 of 2019, and January 1, 2023 falls in week 52 of 2022. This is standard ISO 8601 behavior, not a bug.

Does every week have 7 days, starting Monday?

Yes — ISO 8601 weeks always start on Monday and run through Sunday.

Week Number Calculator

Find a date's ISO 8601 week number.

Date

Week 37

ISO week-numbering year 2026

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How it works

1
Pick a date

Use the date picker.
2
See its ISO week number

Week 1 is the week containing the year’s first Thursday.

FAQ

Why does the "week-numbering year" sometimes differ from the calendar year?: Near a year boundary, a date can belong to the adjacent year’s week — e.g. December 31, 2018 falls in week 1 of 2019, and January 1, 2023 falls in week 52 of 2022. This is standard ISO 8601 behavior, not a bug.
Does every week have 7 days, starting Monday?: Yes — ISO 8601 weeks always start on Monday and run through Sunday.