Does the week start on Monday or Sunday? There's no single answer — it genuinely depends on where you are and which standard you use. Both are "correct." Here's the breakdown.
For week numbering, see what week is it (week numbers explained) or the week numbers reference.
The two conventions
- Monday start — used by most of Europe and much of the world, and it's the international standard (ISO 8601). In this system the weekend (Saturday + Sunday) sits together at the end of the week.
- Sunday start — used in the United States, Canada, Japan, and several other countries. Calendars and apps there usually show Sunday in the first column.
So your phone, your wall calendar, and a colleague abroad may all disagree — and none of them is wrong.
Why the difference?
- Religious tradition. In the Jewish and Christian traditions, Sunday is the "first day" of the week (the Sabbath being Saturday), which influenced the Sunday-start convention.
- The working week. The Monday-start convention reflects the modern working week, grouping the two rest days into a single "weekend." This is what ISO 8601, the international date/time standard, adopted.
Why it matters: week numbers
The start of the week affects week numbering. Under ISO 8601 (Monday start), week 1 is the week containing the first Thursday of the year (equivalently, the week containing 4 January). Systems that start the week on Sunday can number weeks differently — so the "current week number" can vary by system. See week numbers explained.
Quick answers
Does the week start on Monday or Sunday? Both are used: the international ISO 8601 standard starts on Monday; the US, Canada, and Japan start on Sunday.
What is the first day of the week in the US? Sunday.
What does ISO 8601 say? It defines Monday as the first day of the week.
Why do calendars differ? Because of differing religious traditions and standards — Sunday-start (US-style) vs Monday-start (ISO/European).