Week number & day of year

ISO 8601 defines week numbering precisely, which matters because the intuitive definitions disagree at year boundaries.

The two rules

• Weeks run Monday to Sunday.
• Week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January.

The consequence is that up to three days at either end of a calendar year belong to a neighbouring ISO year. 1 January 2021 was a Friday, so it fell in week 53 of ISO year 2020. This tool reports the ISO year alongside the week for exactly that reason.

53-week years

Most years have 52 ISO weeks; some have 53. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Retail and payroll calendars built on 52 weeks have to absorb an extra week periodically, which is why some companies report a “53-week year”.

Other conventions exist

North American business calendars often start weeks on Sunday and define week 1 as the one containing 1 January. That numbering can differ from ISO by one for much of the year — worth checking before quoting a week number to someone in a different country.