ISO 8601 week numbers, and how they differ from US weeks

If two people look up the same date and get different week numbers, neither is wrong. They are using different rules, and both rules are in real use.

Why two systems disagree

Everyone agrees a year has weeks. Nobody agrees which one is week 1, and that is where the two numbers come from.

ISO 8601, used across Europe and by most software, starts weeks on Monday and gives week 1 to the first week with four or more of its days in the new year. That is usually described as "the week containing the first Thursday", which is the same rule said differently: Thursday is the fourth day of a Monday-start week.

The US convention starts weeks on Sunday and gives week 1 to whatever week holds 1 January. It is easier to explain and it produces a 53rd week far more often.

The two can differ by a full week for the same date, which is exactly the sort of thing that goes unnoticed until two reports disagree at a year end.

The four rules, and why the Thursday one is not arbitrary

Thursday is not a magic day. It is the fourth day of a week that starts on Monday, so "the week containing the first Thursday" and "the first week with four or more days in the new year" are two ways of saying one thing.

That equivalence dies the moment weeks do not start on Monday. The fourth day of a Sunday-start week is Wednesday. Keep the Thursday pivot with a Sunday start and week 1 can hold only three days of the new year, which several large retailers do deliberately for their reporting weeks.

So there are four rules in real use, not one:

• First week with 4+ days in the new year. The majority rule. With weeks starting Monday this is exactly ISO 8601.
• Week containing the first Thursday. ISO's pivot kept whatever day your week starts on.
• First week fully inside the new year. Week 1 never begins in December. Common in US federal payroll and in broadcast and retail calendars.
• Week containing 1 January. The simplest to explain, and the one that most often produces a week 53.

Combined with the seven possible week-start days, that is twenty-eight ways to number a year. The calendar does all of them, and shows three years side by side so you can see the effect rather than take it on trust.

A worked example

1 January 2026 is a Thursday. With weeks starting Sunday, the Thursday rule puts week 1 on 28 Dec 2025, holding only three days of 2026. The four-day rule puts it on 4 Jan 2026. A whole week apart, for the same year, on the same calendar.

Neither is a mistake. Which one is right depends entirely on whose reporting week you have to match.

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