Week Numbers Starting Monday

Updated: May 2026

ISO 8601 mandates that weeks start on Monday. This is not just a convention — it has precise effects on which week number is assigned to every date in the year, and it differs from the Sunday-start system used by default in the United States.

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Why ISO weeks start on Monday

The ISO 8601 standard, adopted by the International Organization for Standardization, aligns weeks with the business week used throughout most of Europe and by international industry. Monday as the first day of the week reflects the working week structure where the weekend (Saturday and Sunday) falls at the end, not at the beginning.

This choice also makes week boundaries more natural for planning. A Monday-start week groups a complete set of working days together and separates the weekend as a clear transition point between two consecutive weeks.

The ISO 8601 week day numbering runs 1 (Monday) through 7 (Sunday). This contrasts with the US convention where 0 or 1 represents Sunday. JavaScript's Date.getDay() uses 0 for Sunday, which requires adjustment when computing ISO week numbers.

How Monday start affects week numbers

When a week starts on Monday, the ISO week boundary falls between Sunday and Monday. A date that is a Monday is always the first day of its ISO week. A Sunday is always the last day. This has several practical consequences:

The Monday of any ISO week is always in the same ISO week as the Sunday of that week. There is no split where Monday is in week N and Sunday is in week N−1.
At year end, if December 31 is a Monday, Tuesday, or Wednesday, it belongs to the first ISO week of the following year — because the Thursday of that same week falls in January.
The Monday start ensures that the day-of-week numbering (1–7) maps perfectly to the relative position within the week without any offset calculation.

Configuring Monday start in common tools

Many tools default to a different start day but allow configuration:

Google Calendar — Settings → View options → "Start week on" → Monday.
Microsoft Outlook — File → Options → Calendar → "First day of week" → Monday.
Excel — Use ISOWEEKNUM() instead of WEEKNUM(). ISOWEEKNUM() always uses Monday start and the ISO week 1 definition.
Python — date.isocalendar() returns (ISO year, ISO week, ISO day) with Monday = 1, always following ISO 8601.
SQL (MySQL) — WEEK(date, 3) uses mode 3, which gives ISO weeks with Monday start.

Finding the Monday of any given ISO week

To find the exact Monday (first day) of a given ISO year and week number, the standard algorithm is:

Locate January 4 of the ISO year — it is always in week 1.
Find the Monday on or before January 4 — this is the Monday of week 1.
Add (week − 1) × 7 days to reach the Monday of your target week.

The Flowfiles ISO week calculator does this automatically. Enter any ISO year and week number to get the Monday date (and the full Monday–Sunday range) instantly.