Week Numbers: ISO 8601 vs US Week Numbering
ISO 8601 weeks start on Monday and week 1 is the week with the year’s first Thursday. The common US style starts weeks on Sunday and calls the week containing January 1 week 1. Around New Year the two disagree.
The two rules
| ISO 8601 | US style | |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 is | The week with the first Thursday (the week containing January 4) | The week containing January 1 |
| Days at the turn of the year | Can belong to the last week of the previous year, or week 1 of the next | Always belong to their own calendar year |
| Weeks in a year | 52 or 53 | 53 or 54, with short first and last weeks |
Worked examples
| Date | ISO 8601 | US style |
|---|---|---|
| Thu Dec 31, 2026 | 2026, week 53 | Week 53 |
| Fri Jan 1, 2027 | 2026, week 53 | Week 1 |
| Sun Jan 3, 2027 | 2026, week 53 | Week 2 |
| Mon Jan 4, 2027 | 2027, week 1 | Week 2 |
| Fri Dec 31, 2027 | 2027, week 52 | Week 53 |
January 1, 2027 is a Friday. Its week has only three days in 2027, so ISO counts it as the last week of 2026. That makes 2026 a 53-week year in ISO numbering. In the US style the same Friday is in week 1, and a new week starts two days later on Sunday, January 3.