Week 1, 2026

1. Mon29Dec
2. Tue30Dec
3. Wed31Dec
4. Thu1Jan
5. Fri2Jan
6. Sat3Jan
7. Sun4Jan

ISO week

2026-W01

Copy as

CW1 (29 Dec – 4 Jan)

Quarter

Q1 2026

Weeks in 2026

53

US week number

1

Week 1 is also written KW 1 (Kalenderwoche) or CW 1. ISO weeks start on Monday, and week 1 is the week with the year’s first Thursday. US calendars start weeks on Sunday and count the week with 1 January as week 1, so they give Monday 29 Dec to Saturday 3 Jan the same number: week 1.