ISO week calendar
Week 1 2026
ISO week 1 of 2026 runs from Monday, December 29, 2025 to Sunday, January 4, 2026.
Date Time Encyclopedia answer, with calculation details and assumptions below.
ISO weeks start on Monday and end on Sunday. Week 1 is the week containing the first Thursday of the ISO week-numbering year.
Snapshot
Week 1 2026
ISO week 1 of 2026 runs from Monday, December 29, 2025 to Sunday, January 4, 2026.
ISO weeks start on Monday and end on Sunday. Week 1 is the week containing the first Thursday of the ISO week-numbering year.
- Week starts
- Monday, December 29, 2025
- Week ends
- Sunday, January 4, 2026
- ISO year
- 2026
- Weeks in ISO year
- 53
ISO week 1, 2026
- MonDec 29, 2025Week start
- TueDec 30, 20252026-W01
- WedDec 31, 20252026-W01
- ThuJan 1, 20262026-W01
- FriJan 2, 20262026-W01
- SatJan 3, 20262026-W01
- SunJan 4, 2026Week end
Method
Find the Monday of ISO week 1, then add seven days for each week after it.
ISO week context
Week 1 2026 uses the ISO week-date system: weeks start Monday, end Sunday, and week 1 is tied to the first Thursday of the ISO year.
Compare nearby ISO weeks
Adjacent week pages help verify handoffs between reporting periods, sprints, payroll periods, and weekly plans.
Related Lookups
FAQ
When does this ISO week start?
Week 1 of 2026 starts on Monday, December 29, 2025.
When does this ISO week end?
Week 1 of 2026 ends on Sunday, January 4, 2026.