Week-number guide

What is a week number?

A week number assigns a number to a seven-day period so schedules can refer to “Week 12” or “Week 35” instead of repeating date ranges.

Why the system matters

Different calendars can define both the start of the week and Week 1 differently. That is why a week number by itself can be ambiguous in international communication.

ISO is the default on this site

All year, month, week, date, PDF, and ICS pages use ISO 8601 as the primary system: Monday through Sunday, with Week 1 containing the first Thursday.

US-style comparison

Date pages and the calculator additionally show a Sunday-start US-style number, making differences visible instead of silently assuming one convention.