Week 1 is the week with the first Thursday
ISO 8601 weeks start on Monday, and week 1 of a year is the week containing that year's first Thursday. Equivalently, it is the week containing 4 January. That single rule produces all the behaviour people find surprising.
It means 1 January is often not in week 1. If it falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year: week 52 or 53. And 29, 30 or 31 December often belong to week 1 of the next year.
Any calculation that takes the day of the year and divides it by seven gets those weeks wrong, and it does so several times a decade. It usually goes unnoticed until a programme dated in week numbers meets a programme dated in calendar dates.
Some years have 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Everything else has 52. That produces a 53-week year roughly every five or six years on an irregular cycle, and it is the thing that breaks any schedule built on the assumption of 52.
Payroll feels it first. A weekly payroll in a 53-week tax year runs an extra pay period, which needs handling on its own terms rather than being squeezed into the annual figures.
Where week numbers get used
Construction and manufacturing programmes run on them almost universally in Europe. A delivery date of "week 34" is unambiguous across languages and date formats in a way that 08/09 is not, given that half the world reads that as September and half as August.
Steel and joinery lead times, factory slots, shutdown planning, holiday shutdowns and site programmes are all typically quoted in week numbers. So is a lot of retail and logistics planning.
Not everybody uses ISO
The United States and Canada commonly number weeks from the week containing 1 January, with weeks starting on Sunday. That gives a different number for the same date, often by one, and always at the start and end of the year.
This page uses ISO 8601 throughout, which is the standard across the UK and Europe. If you are reconciling against a US system, expect the numbers near the year boundary to disagree and check which convention the other side is using before assuming an error.
The full year view
Switch on the year list to get every week of a year with its Monday and Sunday, which is the thing worth printing or pinning up. The current week is highlighted.
Common questions
How is week 1 of the year defined under ISO 8601?
As the week containing the first Thursday of the year, which is the same as saying the week containing 4 January. Weeks run Monday to Sunday.
Why is 1 January sometimes in week 52 of the previous year rather than week 1?
Because week 1 is defined by the first Thursday rather than the first day. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year, week 52 or 53, rather than week 1 of the new one.
Do all years have 53 weeks?
No. A year has 53 ISO weeks only when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Every other year has 52, and a 53-week year comes round roughly every five or six years on an irregular cycle.
Why does a 53-week year matter for payroll?
A weekly payroll in a 53-week tax year runs an extra pay period, which needs handling on its own terms rather than being squeezed into the usual annual figures.
Does the United States number weeks the same way as the UK?
No. The United States and Canada commonly number weeks from the week containing 1 January, with weeks starting on Sunday, which gives a different number for the same date, often by one, and always at the start and end of the year.
Why are week numbers used so heavily in construction and manufacturing?
Because a delivery date of "week 34" is unambiguous across languages and date formats, unlike a date such as 08/09, which half the world reads as September and half as August.