Week 1 of 2026
December 29 – January 4, 2026
| Monday | December 29, 2025 | Mon · week starts |
| Tuesday | December 30, 2025 | |
| Wednesday | December 31, 2025 | |
| Thursday | January 1, 2026 | |
| Friday | January 2, 2026 | |
| Saturday | January 3, 2026 | |
| Sunday | January 4, 2026 | Sun · week ends |
All weeks of 2026
2026 is an ISO year with 53 weeks.
About ISO week 1 of 2026
ISO 8601 weeks start on Monday and end on Sunday. Week 1 of 2026 begins on Monday, December 29, 2025 and ends on Sunday, January 4, 2026. Week numbering follows the international standard: week 1 of a year is the week containing the first Thursday of January, which is why some calendar dates near January 1st belong to the previous or next ISO year.
The ISO week system is the standard for business reporting across Europe and much of the world. The US convention differs: weeks start on Sunday and week 1 contains January 1st, so US week numbers can be one ahead of or behind ISO numbers near year boundaries.
Frequently asked questions
What dates are in week 1 of 2026?
Week 1 of 2026 spans December 29 – January 4, 2026: Monday December 29 through Sunday January 4.
When is week 1 of 2026?
Week 1 of 2026 started 302 days ago, on Monday, December 29, 2025.
Why does 2026 have 53 weeks?
2026 is a "long" ISO year with 53 weeks. This happens when the year starts on a Thursday, or on a Wednesday in a leap year — the extra days spill into a 53rd numbered week.