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Week number calculator
A week starts on Monday, and week 1 holds the first Thursday.
An ISO week starts on Monday. 2026-01-01 is week 1, and that Monday is 2025-12-29.
Week
1
2026-W01 · 2025-12-29 → 2026-01-04
- ISO year
- 2026
- Monday
- 2025-12-29
- Sunday
- 2026-01-04
- Weekday
- Thursday
A week runs from Monday through Sunday. Week 1 is the week that holds the year’s first Thursday. A late December date can be week 1 of the next year, and 1 January is not always week 1. There is no holiday calendar.
Why this exists
Reports and calendars meet on a week number, not on the first day of January. 2026-01-01 is 2026-W01, and its Monday is 2025-12-29.
The date-gap page counts days between two dates. This page names the week that holds one date, and the Monday and Sunday around it.
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01How to use it
Look up a date, or type a year and a week to see that Monday.
The large number is the week. In week mode it is the Monday.
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02What you get
The tiles show the ISO year, the Monday, the Sunday, and the weekday of the chosen date.
The link keeps the date, or the year and the week.
Entered1 January 2026
Date 2026-01-01 Week 2026-W01 Monday 2025-12-29 Sunday 2026-01-04 -
03Worth knowing
The business-day page counts open days and drops weekends. This page only names the week.
The result stays in this browser.
Questions
- QWhat week is 2026-01-01?
- A2026-W01. Thursday falls on 1 January, so that week is week 1.
- QWhat is that Monday?
- A2025-12-29.
- QWhen is week 2?
- AMonday 2026-01-05 through Sunday 2026-01-11.
- QIs 1 January always week 1?
- ANo. Week 1 is the week with the first Thursday.
How this page works it out
An ISO week starts on Monday and ends on Sunday. Week 1 is the week that holds the year’s first Thursday. 2026-01-01 is a Thursday, so it is week 1 of 2026, and that week’s Monday is 2025-12-29 and its Sunday is 2026-01-04.
1 January is not always week 1. 2025-12-29 is also 2026-W01. 2026-01-05 is week 2, and 2026-12-31 is week 53. 2021-01-01 belongs to week 53 of 2020.
A holiday has no name here. The count of calendar days and the count of business days are different pages. This one only names the week.