Calendar date
2025-12-29
ISO week date
2026-W01-1
A December date can belong to the next ISO year.
ISO Week Date System
A practical guide to ISO week years, week numbers, weekdays, and the New Year edge cases that make week dates differ from calendar dates.
- Weeks start
- Monday
- Week 1 rule
- First Thursday
- Weekdays use
- 1 to 7
What is the ISO week date system?
The ISO week date system is a calendar system where weeks start on Monday and Week 1 is the week containing the first Thursday of the year.
ISO week dates are commonly written as a week-year, week number, and weekday, such as 2026-W18-7 for Sunday, May 3, 2026.
How ISO week numbers work
Each ISO week belongs to an ISO week-year. Around New Year, that week-year can differ from the calendar year, which is why December 29 can be Week 1 of the next ISO year.
| Rule | ISO behavior |
|---|---|
| Week start | Monday |
| Week end | Sunday |
| Week 1 | The week containing January 4, also the first Thursday of the year |
Why Week 1 can start in December
If the week containing January 4 starts in late December, those December dates belong to ISO Week 1 of the next ISO year. This is expected, not an error in the calendar.
Why some years have 53 weeks
An ISO year has 53 weeks when January 1 is Thursday, or when a leap year starts on Wednesday. 2026 is a 53-week ISO year.
ISO vs US-style week numbers
ISO weeks start on Monday. US-style weeks in WhatWeek start on Sunday and count the week containing January 1 as Week 1.
Examples around New Year
| Calendar date | ISO week date | Why it matters |
|---|---|---|
| 2025-12-29 | 2026-W01-1 | A December date can belong to the next ISO year. |
| 2026-12-28 | 2026-W53-1 | 2026 has an ISO Week 53. |
| 2027-01-03 | 2026-W53-7 | An early January date can still be in the previous ISO year. |
| 2027-01-04 | 2027-W01-1 | The 2027 ISO year begins on Monday, January 4. |
Calendar date
2026-12-28
ISO week date
2026-W53-1
2026 has an ISO Week 53.
Calendar date
2027-01-03
ISO week date
2026-W53-7
An early January date can still be in the previous ISO year.
Calendar date
2027-01-04
ISO week date
2027-W01-1
The 2027 ISO year begins on Monday, January 4.
Try the ISO week calculator
Convert a date to its ISO week date, turn an ISO week number back into a date range, or open the full week number calculator.
Code examples
For implementation details, see the JavaScript week number guide or use the WhatWeek API for stable JSON responses.
FAQ
Do ISO weeks always start on Monday?
Yes. ISO weeks always start on Monday and end on Sunday.
Can an ISO week-year differ from the calendar year?
Yes. Dates near January 1 can belong to the previous or next ISO week-year.
How do I convert a date to an ISO week date?
Use the date to week converter to enter a calendar date and get the ISO week-year, week number, weekday, and Monday-to-Sunday date range.
Why does 2026 have 53 ISO weeks?
2026 starts on a Thursday, which gives the ISO week-year a Week 53. ISO Week 53 of 2026 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.