ISO week numbers: why January 1 can be in week 53
In the ISO 8601 system, weeks run from Monday to Sunday and week 1 is the week that contains the year's first Thursday. The first days of January can therefore belong to the last week of the previous year. January 1, 2027 is a Friday and falls in week 53 of 2026; week 1 of 2027 starts on Monday, January 4.
The rule
- A week starts on Monday.
- Week 1 is the week with the year's first Thursday in it, which is the same as the week that contains January 4.
- A week belongs to the year its Thursday is in, so a week is never split between two years.
Around New Year
| Date | ISO week |
|---|---|
| Mon, Dec 29, 2025 | 2026-W01 |
| Thu, Jan 1, 2026 | 2026-W01 |
| Mon, Dec 28, 2026 | 2026-W53 |
| Fri, Jan 1, 2027 | 2026-W53 |
| Sun, Jan 3, 2027 | 2026-W53 |
| Mon, Jan 4, 2027 | 2027-W01 |
| Mon, Dec 31, 2029 | 2030-W01 |
2026-W53 means week 53 of ISO year 2026. Use the ISO year, not the calendar year, when you label or sort by week, because the two can differ for up to three days at either end of the year. Python's documentation gives a similar case: 2004 began on a Thursday, so ISO week 1 of 2004 started on Monday, December 29, 2003.
Which years have 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, or when it's a leap year that starts on a Wednesday. Between 2015 and 2040 that happens in 2015, 2020, 2026, 2032 and 2037. December 28 is always in the last week of its ISO year, so its week number tells you whether a year has 52 or 53 weeks.
Why a spreadsheet can show a different number
Excel's WEEKNUM function uses another system by default: the week containing January 1 is week 1, and weeks begin on Sunday. So WEEKNUM puts January 1, 2027 in week 1, while ISO puts it in week 53 of 2026. Microsoft's documentation says WEEKNUM(date, 21) follows ISO 8601 instead and calls it the method "commonly known as the European week numbering system". When you share week numbers with other teams, say which system you mean.
Week of the month
The week number calculator also shows the week of the month. ISO 8601 numbers weeks within a year, not within a month, so Daycount uses a simple convention for this: the week containing the 1st is week 1, and weeks start on Sunday. Monday, October 5, 2026 is in the 2nd week of October and in ISO week 41.
For the number of days between two dates rather than weeks, see how to count the days between two dates.
Look up the ISO week of any date →