Week 1 2026: December 29–4, 2026
Week 1 starts on Monday, December 29, 2025 and ends on Sunday, January 4, 2026.
Quick answer
Week 1 of 2026 is the seven-day ISO week from December 29, 2025 to January 4, 2026. ISO weeks always run from Monday to Sunday.
Use week 1 in professional planning
This permanent week reference can be used in project plans, payroll notes, weekly reports, production schedules and deadline documents. Share the week number together with the ISO year and exact date range to avoid ambiguity around New Year.
For calculations that span more than one week, use the weeks between dates calculator. For work schedules, compare the same range with the business days calculator or open the project timeline calculator.
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FAQ
What are the dates for week 1 of 2026?
The dates are December 29, 2025 through January 4, 2026.
What day does this week start?
It starts on Monday.
What day does this week end?
It ends on Sunday.
How should I write this week in a report?
Use “ISO week 1 of 2026 (Dec 29–Jan 4, 2026)” so the week-year and date range are both clear.