Week number and day of week

What the result means

ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the Gregorian year — equivalently, the week that contains 4 January. Early January can still be week 52 or 53 of the previous ISO year, which is why a timesheet labelled “week 1” is not always the week that contains New Year’s Day.

How it is calculated

ISO week-year is the year of the Thursday of that week. Week 1 contains 4 January

• Weekday: Sunday through Saturday in English.

The algorithm finds the Thursday of the current week, then counts weeks from the first Thursday of that ISO year.

Worked example

1 January 2026

New Year’s Day 2026 is a Thursday.

Weekday

Thursday

ISO week

Week 1 of 2026

Because Thursday sits in week 1, 1 January 2026 is week 1 — unlike years when 1 January is a Friday–Sunday.

Australian business calendars usually mean ISO weeks

Payroll cycles, ISO week-date documents and most Australian project plans treat Monday as the first day of the week. That matches ISO 8601 and this page. It does not match the US convention that starts the week on Sunday and forces 1 January into week 1 of the new calendar year.

If a US timesheet or a school term list disagrees with the number here, check which rule they published. The weekday name is not in dispute; the week number is.

The week-year can differ from the calendar year

Wednesday 31 December can sit in ISO week 1 of the next year, because that week’s Thursday is 1 January. Conversely, Friday 1 January can sit in week 53 of the previous ISO year. Filing a document as “2027-W01” on that Friday would be the wrong ISO week-year.

Questions