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Week number

38ISO week of 2026

Saturday, September 19, 2026

ISO week begins
September 14, 2026
ISO week ends
September 20, 2026
Sunday-start week
38 of 2026
Sunday-start dates
September 13, 2026 to September 19, 2026

ISO starts Monday; week one contains the first Thursday. Sunday-start uses the week containing January 1.

How this count works

ISO weeks start on Monday. Week one contains the year’s first Thursday. The Sunday-start comparison uses the week containing January 1 as week one.

A date near New Year can belong to a neighboring week-year. Sunday-start is a stated convention here; calendars used in the US do not all number weeks the same way.

See how we count for the calendar conventions behind the answer.

Next, if you need it

For the date’s position within its calendar year, see day of the year.

For a number of weeks before or after it, use the date calculator.

A worked example

January 1, 2021 is in ISO week 53 of 2020. The Sunday-start comparison puts it in week 1 of 2021.