Why Do Weeks Start on Different Days?

Both conventions trace back to the same 7-day cycle (see why the week has 7 days at all), but disagree about which day anchors it. The Sunday-first convention has religious roots — many Christian traditions treat Sunday as the first day, tied to it being the day of the resurrection, and it's also consistent with the biblical seven-day creation account ending with rest on the seventh day, Saturday.

Where the Monday-first convention comes from

ISO 8601, the international standard behind most of the date formatting on this site, explicitly defines Monday as day 1 and Sunday as day 7 — a deliberate choice made to align the calendar week with the standard working week (Monday through Friday) followed by the weekend, which is the more common practical framing outside the US.

It's genuinely inconsistent even within English-speaking countries

The US, Canada, and a few other countries default to Sunday-first in everyday calendars and software; the UK, most of Europe, and international standards default to Monday-first. This is exactly why so many calendar apps and libraries include an explicit "week starts on" setting — there's no universal default that's correct for every audience, only a US convention and an ISO/international one.

Why this matters for developers specifically

JavaScript's Date.prototype.getDay() returns 0 for Sunday, following the US convention — but the ISO week date system and week-number calculations used throughout this site follow the Monday-first convention instead, since that's what the ISO 8601 standard specifies. Code that mixes both assumptions without converting between them is a common, easy-to-miss source of off-by-one bugs in calendar UI and week-numbering logic.