A week number is useful when you plan in seven-day blocks. But “week 1” only makes sense once everyone agrees on the rules. This site uses the ISO week system throughout the live dashboard.
Three rules to remember
ISO weeks start on Monday. Week 1 contains the first Thursday of the year. Every week belongs to one ISO week-year, even if its dates cross a December–January boundary.
The Thursday rule also means that week 1 contains January 4, and that at least four of its days fall in the new calendar year. You can use whichever of these equivalent descriptions is easiest to remember.
Why the year can look wrong
Friday, January 1, 2021 was in ISO week 53 of 2020. The date belonged to the 2021 calendar, but its Monday–Sunday week had most of its days in 2020. By contrast, Tuesday, December 31, 2019 belonged to ISO week 1 of 2020.
Write the week-year too: “2020-W53” is clearer than “week 53” when a plan crosses New Year.
Where the extra week comes from
Fifty-two full weeks account for 364 days. A calendar year has one or two more days, so the relationship between weekdays and the calendar shifts each year. Under ISO rules, some week-years contain 53 weeks. The number is not a measure of how many full weeks have elapsed since January 1.
How to use it in everyday planning
When you share a week-based schedule, say that you use ISO weeks. If your team uses Sunday-first calendars, a fiscal calendar, or another week-number system, write actual start and end dates alongside the number. Our printable calendar can start on Monday or Sunday, but changing that display setting does not change the ISO calculation on the home page.
Reference and worked boundary examples: MDN’s weekOfYear documentation. General standard: ISO 8601 date and time format.