Short answer
The ISO week date explained
A clear guide to Monday-based weeks, week 1 and the ISO week-year.
Short answer
ISO weeks contain seven days from Monday to Sunday; week 1 contains the year’s first Thursday.
How it works
Equivalently, week 1 contains 4 January and at least four days of the new year.
Three ISO rules
ISO weeks start on Monday, end on Sunday and always contain seven days. Week 1 contains 4 January, or equivalently the first Thursday of the year. The year of that Thursday is the ISO week-year.
A Monday start by itself does not guarantee ISO numbering. Other calendars may count the week containing 1 January as week 1 or start on Sunday. There is no single universal US or business week system: check both the first weekday and the week-1 rule. Fiscal and 4-4-5 calendars need their own definition.
4-4-5 is a separate fiscal calendar
Four, four and five weeks form a 13-week quarter; four quarters total 52 weeks. Define the year start, weekday and placement of any 53rd week explicitly. ISO week-years do not provide these business rules. A deliberately chosen 52-week year starting Monday 2 February 2026 ends Sunday 31 January 2027; this example is not a universal fiscal standard.