A week number seems like a simple concept — until you discover that different systems count it differently, and the same calendar day can have a different week number depending on which standard is used.
The ISO 8601 system
Under ISO 8601, the week starts on Monday, and the year's first week is the one containing the first Thursday of January (equivalently: the first week contains January 4). That means January 1 sometimes falls into the last week (52 or 53) of the previous year.
Why "the first Thursday" specifically
The rule guarantees that most days of the first week (at least 4 of 7) fall within the new year — a deliberate compromise that avoids situations where "the first week" consists of just one or two days of the new year.
Why US retail uses a completely different week system
Beyond the common Sunday-start convention, most large US retailers and consumer brands report finances on a "4-5-4" fiscal calendar defined by the National Retail Federation, which groups every quarter into 4 weeks, then 5, then 4, always ending fiscal weeks on a Saturday. That calendar's week numbers don't correspond to ISO week numbers at all, which is a frequent source of confusion when comparing US retail data to internationally reported figures.
Why you'd need this
- Reconciling week numbering in reports between systems using different standards.
- Correctly labeling work weeks in planners and calendars.
- Avoiding parsing errors with week numbers coming from external APIs or files.
Why Some Years Have a 53rd Week
Most years have 52 ISO weeks, but a year that starts on a Thursday (or a leap year that starts on a Wednesday) has 53 weeks — this follows from the fact that 52 × 7 = 364 days, one or two days short of a calendar year, and these "extra" days periodically form an additional week. Code that hard-codes a maximum of 52 weeks will eventually run into this edge case.