ISO week numbers explained, and why some years have 53.
ISO weeks start on Monday and week 1 holds the year's first Thursday, so early January can be week 52 or 53. How it differs from the US way.
By Suraj Kumar3 min read
Week 45 means one thing in Europe and sometimes another in the US. Here are the two ways.
The ISO week.
ISO 8601, used across Europe and in most planning software, starts weeks on Monday, and week 1 is the week holding the year's first Thursday (so it always holds 4 January). The first days of January can therefore belong to the year before: 1 January 2027 is in week 53 of 2026.
Years with 53 weeks.
A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year; 2026 is one. The answer says how many weeks the year has, and the Monday and Sunday the week runs from and to.
The US way.
Many US calendars start weeks on Sunday, and call the week holding 1 January week 1, however short it is. Choose US under More options for that numbering; the answer gives the other as well.
Further reading
Topics: what week is it · week number · current week number · iso week · week of the year · 53 week year
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