Why Do Some Countries Start the Week on Sunday and Others on Monday?
Open a calendar app in the United States and the week starts on Sunday. Open one in the United Kingdom, Germany or India and it starts on Monday. Same seven days, same order — Sunday, Monday, Tuesday, and so on — just a different starting point. Neither is "wrong"; they come from two different traditions that never fully merged.
Two traditions, one week
The seven-day week itself traces back to the ancient Babylonians and, separately, to the Jewish calendar, where the week culminates in the Sabbath on Saturday and the new week begins the next day — Sunday. Early Christian tradition inherited this pattern, which is part of why Sunday, the day of worship and rest, functioned culturally as the start of the week across much of the Western world for centuries.
The Monday-first convention comes from a more modern, secular source: the ISO 8601 international standard, adopted in the late 20th century to make dates and calendars easier to exchange consistently across computer systems, business schedules and international trade. ISO 8601 explicitly defines Monday as day 1 of the week, largely because it aligns with the working week — Monday to Friday — followed by the weekend. Most of Europe, and countries that adopted ISO-based scheduling for business and government systems, followed suit.
Why the US and a few others kept Sunday-first
The United States, Canada (in everyday use, though Canadian government systems often follow ISO), and a handful of other countries kept the older Sunday-first convention in day-to-day calendars, largely out of continuity — that's simply what people grew up seeing on paper calendars, planners and, later, digital calendar apps. India generally follows Sunday-first too, both from British-era convention in some contexts and independent cultural practice, though Monday-first is also common in business settings influenced by ISO scheduling.
Does it actually matter?
For most day-to-day purposes, no — the days themselves and their order never change, only which one visually appears first in a grid. It matters more in software and business contexts: week numbers (like the ISO week number shown on this site) are defined relative to a Monday start, so a "week 37" calculation assumes Monday is day one, regardless of how your calendar app happens to display the grid.
On Tomorrow Calendar, our monthly calendar follows each supported country's typical convention — for example, a Sunday-first layout for the United States and a Monday-first layout for the United Kingdom — while our week number tool always uses the ISO-8601 standard (Monday-start) internally, since that's the definition used in business, government and international scheduling worldwide.
Curious how week numbers or day-of-year counts are worked out? See our guide on how the day of the week is calculated.