ISO and US Week Numbers
| Rule | Week starts | Week 1 contains |
|---|---|---|
| Our US convention | Sunday | January 1 |
| ISO convention | Monday | January 4 / the first Thursday |
ISO weeks start on Monday
ISO week 1 contains January 4 and the first Thursday of the calendar year. A week has a week-year as well as a number. January 1, 2027 belongs to ISO week 53 of 2026, while Monday, January 4 starts ISO week 1 of 2027. January 1, 2028 belongs to ISO week 52 of 2027.
The US convention used here
Our Sunday-start week 1 contains January 1. A Sunday–Saturday week that crosses New Year belongs to the year containing that January 1. This is an explicit site convention; US organizations can use other systems. December 27, 2026 through January 2, 2027 is US week 1 of 2027.
Use the same rule in the file and your workflow
When week numbers are enabled on a monthly calendar, Monday-start files use ISO week numbers and Sunday-start files use our January 1 convention. Multi-month overviews do not include numbered week rows. Switching week start changes the grid boundaries and can change the week number; it never changes the civil date. The lookup tool reports both the number and its week-year so a bare “week 1” cannot hide a year boundary.
Prepared by Print Grid Calendar. Calendar sources and methodology.