ISO Week Numbers Explained: The Thursday Rule and Year-Crossing Weeks
How ISO 8601 defines week 1 by the year's first Thursday, why some weeks span two calendar years, and how to spot a 53-week year.
What is an ISO week number?
ISO week numbering, defined by the ISO 8601 standard, runs Monday through Sunday and splits a year into 52 or 53 weeks. Week 1 is defined as the week containing the year's first Thursday, which is equivalent to the week containing January 4. Most calendar apps and business systems that show a "week number" are following this same standard.
Why does the "Thursday rule" decide week 1?
Thursday sits in the middle of a Monday-to-Sunday week, so whichever calendar year owns that Thursday also owns the majority (4 or more) of that week's days. January 4 always falls inside the week containing the year's first Thursday, no matter how the days line up, which is why the Thursday rule and the "week containing Jan 4" rule always point to the same week.
When does the year-end roll into next year's week 1?
January 1, 2026 falls on a Thursday, so that week's Monday is December 29, 2025. Because this week contains January 4, it becomes week 1 of 2026 -- meaning December 29-31, 2025 carry the label "2026-W01," a typical case of a week crossing into the following calendar year.
When does the new year fall into last year's final week?
The reverse happens too. January 1, 2027 falls on a Friday, and that week's Thursday lands on December 31, 2026. So January 1-3, 2027 belong to 2026's final week -- week 53, since 2026 is a 53-week year -- carrying the label "2026-W53." Year-crossing weeks can appear at either end of the calendar.
How to spot a 53-week year
A year has 365 days (366 in a leap year), which is 52 weeks plus 1 or 2 extra days, so only years meeting a specific condition stretch to 53 weeks. A year gets 53 weeks when January 1 falls on a Thursday (leap or not), or when it's a leap year and January 1 falls on a Wednesday. This happens roughly once every 5-6 years.
Things to watch for with ISO week numbers
ISO weeks assume a Monday start, so they can disagree with Sunday-start conventions used elsewhere, like the US calendar style. Also, a label like "2026-W01" alone doesn't pin down a specific date -- you need a weekday number (1 through 7) as well to resolve it to an exact calendar day.
Frequently asked questions
What day does an ISO week start on?
Monday, running through Sunday. This can shift the week number compared with Sunday-start conventions.
When does week 1 of 2026 start and end?
Monday, December 29, 2025 through Sunday, January 4, 2026. Since January 1, 2026 is a Thursday, the last three days of December also fall in that week.
How do you tell if a year has 53 ISO weeks?
A year gets 53 weeks when January 1 falls on a Thursday (leap or not), or when it's a leap year and January 1 falls on a Wednesday.