Weeknumber.io

ISO week 2026-W01

Week 1, 2026

Monday, December 29, 2025 – Sunday, January 4, 2026

ISO week 1 of 2026 (2026-W01) runs from Monday, December 29, 2025 to Sunday, January 4, 2026.

This week ended 41 weeks ago. The current week is week 42, 2026.

The 7 days of week 1

DayDateDay of yearUS weekISO date
Monday363 of 365532026-W01-1
Tuesday364 of 365532026-W01-2
Wednesday365 of 365532026-W01-3
Thursday1 of 36512026-W01-4
Friday2 of 36512026-W01-5
Saturday3 of 36512026-W01-6
Sunday4 of 36522026-W01-7

Week 1 at a glance

ISO code
2026-W01 (Monday is 2026-W01-1, Sunday is 2026-W01-7)
Month
December and January
Quarter
Q1 by the ISO rule (its Thursday, January 1, is in Q1); the calendar dates span Q4 and Q1
Day of year
December 29, 2025 is day 363 of 2025; January 4, 2026 is day 4 of 2026
US week number
Monday–Wednesday: US week 53 of 2025; Thursday–Saturday: US week 1 of 2026; Sunday: US week 2 of 2026 (Sunday-start weeks, week 1 contains January 1)
Even or odd
Week 1 is an odd week
Weeks left
After this week, 52 ISO weeks remain in 2026 (53 weeks in total)
Year progress
This week spans New Year

Holidays and key dates

  • Thursday, January 1: New Year's Day – US federal holiday

Why week 1 starts here

Week 1 is the week that contains the first Thursday of 2026 (Thursday, January 1, 2026). That is the same as saying it is the week containing January 4.

That is why week 1 of 2026 starts in December: 3 days of 2025 (December 29–December 31) already belong to ISO year 2026.

Week numbers follow ISO 8601: weeks start on Monday and week 1 contains the first Thursday of the year. US weeks start on Sunday, with week 1 containing January 1.

Questions about week 1, 2026

What are the dates of week 1 in 2026?
Week 1 of 2026 starts on Monday, December 29, 2025 and ends on Sunday, January 4, 2026.
What month is week 1 of 2026?
Week 1 falls in December and January. By the Thursday rule that ISO 8601 uses for years and quarters, it counts as a January week.
How do I get week 1 of 2026 in Excel or Google Sheets?
=ISOWEEKNUM(DATE(2025,12,29)) returns 1. To go the other way, =DATE(2026,1,4)-WEEKDAY(DATE(2026,1,4),2)+1+(1-1)*7 returns the Monday, December 29, 2025.