How is the ISO week number defined?

ISO-8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4). A year has 52 or 53 weeks.

Why can early January be week 52 or 53?

If the first days of January fall before that year's first Thursday, they belong to the last week of the previous year — so 1 January can be week 52 or 53 of the prior week-year.

Week Number Calculator

Free week number calculator. Enter a date to get its ISO-8601 week number, the week-based year and the day of the year. Weeks start on Monday and week 1 contains the first Thursday.

Find the ISO-8601 week number for any date.

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