Find the ISO week number for New Year’s Day 2026.
The ISO 8601 week-numbering system divides the year into 52 or 53 numbered weeks, each running Monday through Sunday. Week 1 is defined as the week containing the year's first Thursday — equivalently, the week containing January 4th — which means the last few days of December can belong to week 1 of the *following* year, and the first few days of January can belong to the *last* week of the *previous* year.
This matters for anything scheduled or reported "by week number" — payroll cycles, project sprints, manufacturing batch codes, and many business calendars all use ISO week numbers rather than calendar months.
Worked example: December 29, 2025 is a Monday, and that Monday-through-Sunday week also contains January 4, 2026 (that Sunday) — so by the ISO rule, December 29-31, 2025 are actually part of Week 1 of 2026, not the final week of 2025. Anyone assuming "late December" always means the previous year's last week would get this specific date wrong.
Because ISO weeks are defined by which year contains that week’s Thursday, not by the calendar month — if December 29-31 fall in a week whose Thursday is already in January, that week belongs to the new year.
No — most years have 52 weeks, but years where January 1st falls on a Thursday (or, in a leap year, a Wednesday) have 53.
No — that naive approach doesn’t account for which day of the week January 1st falls on, and gives wrong results for most years. The ISO definition specifically anchors week 1 to the first Thursday.