Week 1 2026: Dec 29, 2025 – Jan 4, 2026
When is week 1? The exact Monday–Sunday dates of ISO week 1 for every year from 2024 to 2032.
Week 1 of 2026 runs from Monday, Dec 29, 2025 to Sunday, Jan 4, 2026 (Dec / Jan). In 2027, week 1 runs from Monday, Jan 4, 2027 to Sunday, Jan 10, 2027. ISO week code: 2026-W01.
The seven days of week 1
| Date | ISO week date | Day of year | Holidays |
|---|---|---|---|
| Monday, December 29, 2025 | 2026-W01-1 | 363 | — |
| Tuesday, December 30, 2025 | 2026-W01-2 | 364 | — |
| Wednesday, December 31, 2025 | 2026-W01-3 | 365 | New Year's Eve |
| Thursday, January 1, 2026 | 2026-W01-4 | 1 | New Year's Day |
| Friday, January 2, 2026 | 2026-W01-5 | 2 | — |
| Saturday, January 3, 2026 | 2026-W01-6 | 3 | — |
| Sunday, January 4, 2026 | 2026-W01-7 | 4 | — |
Day-of-year numbers and common US and international holidays. Weekday digits in the ISO week date run 1 (Monday) to 7 (Sunday).
Week 1 dates by year
| Year | Week 1 starts (Mon) | Week 1 ends (Sun) | Month |
|---|---|---|---|
| 2024 | Monday, January 1, 2024 | Sunday, January 7, 2024 | January |
| 2025 | Monday, December 30, 2024 | Sunday, January 5, 2025 | Dec / Jan |
| 2026 | Monday, December 29, 2025 | Sunday, January 4, 2026 | Dec / Jan |
| 2027 | Monday, January 4, 2027 | Sunday, January 10, 2027 | January |
| 2028 | Monday, January 3, 2028 | Sunday, January 9, 2028 | January |
| 2029 | Monday, January 1, 2029 | Sunday, January 7, 2029 | January |
| 2030 | Monday, December 31, 2029 | Sunday, January 6, 2030 | Dec / Jan |
| 2031 | Monday, December 30, 2030 | Sunday, January 5, 2031 | Dec / Jan |
| 2032 | Monday, December 29, 2031 | Sunday, January 4, 2032 | Dec / Jan |
All weeks
Frequently asked questions
What month is week 1 in?
Week 1 typically falls in January. The exact dates shift by up to a few days from year to year because week 1 is anchored to January 4, not January 1.
How do I write week 1 as a week code?
The ISO code is the year plus W01 — for example 2026-W01, short form 26W01, or YYWW date code 2601. See week number formats.
Are these dates the same in every country?
Yes for ISO 8601 (Monday-start) numbering, which is what this table shows. US Sunday-start calendars can label the same days with a number one higher — see week start conventions.