Week —
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ISO week-year
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Week runs (Mon–Sun)
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Weeks in that year
Uses the ISO 8601 standard: weeks start on Monday and week 1 is the week containing the year’s first Thursday.
Frequently Asked Questions
How is the ISO week number defined?
Under ISO 8601, weeks begin on Monday and week 1 is the week that contains the first Thursday of the year (equivalently, the week containing 4 January). A year has either 52 or 53 ISO weeks.
Find the ISO 8601 week number for any date, the week-year label and the dates the week runs from.