ISO Week Numbers: Why January 1st Is Sometimes in Week 53

"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 week Real examples: Date ISO Week ISO Week Year December 29, 2025 Week 1 2026 December 30, 2025 Week 1 2026 January 1, 2026 Week 1 2026 December 28, 2026 Week 52 2026 December 29, 2026 Week 53 2026 January 1, 2027 Week 53 2026 Notice: January 1, 2027 belongs to ISO week 53 of 2026 . ISO Week vs US Week The US system is different: Property ISO 8601 US (common) First day of week Monday Sunday Week 1 definition Contains first Thursday Contains January 1 Week year can differ from calendar year Yes No This diverge