Week-number guide
ISO week date system
The ISO week date system makes numbered weeks internationally consistent. Four rules explain almost everything.
An ISO week begins on Monday.
It ends on Sunday and always has seven days.
Week 1 contains the first Thursday of the year.
An ISO week-year has 52 or 53 weeks.
Why can the ISO year differ from the calendar year?
The first days of January can still belong to the final ISO week of the previous year, while the final days of December can already belong to Week 1 of the next ISO year. That is why an ISO date uses a separate week-year.
Equivalent Week 1 rule
Week 1 is also the week containing January 4. These definitions are equivalent because the week containing January 4 always contains the first Thursday.
Try it
Use the week-number calculator to test any date and see its ISO notation plus the US-style comparison.
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