ISO weeks, explained (and why this site runs on them)
What ISO week numbers are, how week 1 is decided, and why a quiz archive is better organised by ISO weeks than by dates.
Every page on this site carries a stamp like 2026 · WK 27. That number isn't ours — it's ISO-8601 week numbering, the international standard that payroll systems, factories, and broadcasters quietly run on. Here's how it works and why we adopted it.
The rules
ISO weeks start on Monday and end on Sunday. Week 1 of a year is the week containing the first Thursday of January — equivalently, the week containing 4 January. Most years have 52 weeks; years starting on a Thursday (or leap years starting on Wednesday) get a week 53.
The odd consequences
Because week 1 is defined by the first Thursday, the 1st of January can belong to the previous year's final week. December 31st, 2025 sits in the same week as January 1st, 2026 — one week, two calendar years, one quiz. ISO numbering resolves the ambiguity by assigning the whole week to the year that owns its Thursday.
Why a quiz archive loves ISO weeks
Dates are ambiguous across formats and time zones; week numbers aren't. 'The week of the 5th' means different spans in different countries, but week 27 of 2026 is exactly Monday 29 June through Sunday 5 July everywhere on Earth. For an archive meant to be cited, linked, and revisited years later, that precision is worth the small learning curve — which, conveniently, this post just handled.