ISO Week Numbers: Why January 1st Is Sometimes in Week 53
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"Week 1 of 2026 starts on December 29, 2025." If that sentence confused you, welcome to ISO week numbers — one of the most counterintuitive corners of date arithmetic. What Is an ISO Week Number? ISO 8601 defines a week as Monday to Sunday. Week 1 of a year is the week containing the year's first Thursday. This means: A year has either 52 or 53 ISO weeks The first days of January can belong to the previous year's last week The last days of December can belong to next year's first