Week 1 of 2026 runs December 29 to January 4
ISO week 1 of 2026 begins on Monday, December 29, 2025 and ends on Sunday, January 4, 2026. It has already passed; the current week is 39. The week covers the turn from December into January, and it also straddles the turn of the calendar year.
What dates are in week 1?
| Weekday | Date | Day of year | Notes |
|---|---|---|---|
| Monday | December 29, 2025 | 363 | |
| Tuesday | December 30, 2025 | 364 | |
| Wednesday | December 31, 2025 | 365 | New Year's Eve |
| Thursday | January 1, 2026 | 1 | New Year's Day |
| Friday | January 2, 2026 | 2 | |
| Saturday | January 3, 2026 | 3 | |
| Sunday | January 4, 2026 | 4 |
Week 1 of 2026 contains New Year's Eve on December 31 and New Year's Day on January 1. The seven dates always run Monday through Sunday, which is the part of ISO 8601 that most often catches people out: a week that starts on Sunday is a different convention and gives a different number.
How ISO week 1 is worked out
ISO 8601 fixes week one as the week containing the first Thursday of January, which is the same as saying the week containing 4 January, or the week holding the majority of its days in the new year. Counting forward from there, week 1 is the 1st Monday-to-Sunday block of 2026, which lands on December 29. Because the block crosses 31 December, some of its dates carry the calendar year 2025 and the rest carry 2026, while the whole week keeps the ISO year 2026. That split is exactly why ISO numbers weeks by their own year rather than by the calendar year.
When is week 1 in other years?
| Year | Monday | Sunday | Weeks in year |
|---|---|---|---|
| 2026 | December 29 | January 4 | 53 |
| 2027 | January 4 | January 10 | 52 |
| 2028 | January 3 | January 9 | 52 |
| 2029 | January 1 | January 7 | 52 |
| 2030 | December 31 | January 6 | 52 |
Week 1 shifts by a day or two each year because a year is 52 weeks plus one or two days, so the Monday that opens it moves from December 29 in 2026 to January 4 in 2027.
How far away is week 1?
Today is Wednesday, September 23, 2026, which sits in ISO week 39 of 2026. Week 1 of 2026 closed 262 days ago. The same week number in the following year is listed in the table above.
Questions
- What dates are in week 1 of 2026?
- Monday December 29, 2025 through Sunday January 4, 2026.
- What week is it now?
- ISO week 39 of 2026. Today is Wednesday, September 23, 2026.
- How many weeks does 2026 have?
- 53. The 2026 week count page explains the rule.
- Do ISO weeks start on Monday or Sunday?
- Monday. A Sunday-first week is a different convention and can give a different week number for the same date.
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