3 min read · Time
Many businesses, schools, and planning tools refer to dates by week number (“week 40”). The international standard, ISO 8601, has two simple rules, but they produce a few results that surprise people every January.
The two rules
Weeks start on Monday.
Week 1 is the week that contains the year’s first Thursday. An equivalent rule: week 1 is the week that contains January 4.
Week numbers are written like 2026-W40, and a specific day like 2026-W40-1 (the Monday of that week).
Why January 1 can be in week 52 or 53
If January 1 falls on a Friday, Saturday, or Sunday, those days belong to the last week of the previous year. For example, January 1–3, 2027 are part of 2026-W53. The reverse also happens: December 29, 2025 was already 2026-W01.
52 or 53 weeks
Most ISO years have 52 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks.
Other week-numbering systems
Some US calendars and spreadsheets start weeks on Sunday and call the week containing January 1 “week 1.” That can put their week numbers one off from ISO. If a colleague’s week number doesn’t match yours, check which system each of you is using.
Try it yourself
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