Why do calendars start the week on different days in different places?
The Real Answer
Calendars start the week on different days because there is no single worldwide convention for which weekday must appear first in a calendar display. Different countries and locales have developed different conventions, and modern software preserves those differences through locale-specific week settings.
International data standards such as Unicode CLDR even include a firstDay setting for different territories so calendar apps can display weeks according to local expectations. Separately, systems that calculate numbered weeks—such as ISO-style week calculations—may use Monday as part of their own rules.
That is why two calendars can contain exactly the same seven-day week but arrange it differently on the page. The difference is mostly one of convention and localization rather than one calendar being more mathematically correct than another.
Week-start rules also should not be reduced to one religion, one country, or one historical explanation. Calendar display conventions, weekday numbering, and rules for determining the first numbered week of a year can all differ independently.
Some calendars start the week on Sunday, others on Monday, and somehow both versions make you feel like you’ve already missed something. You open the calendar to check one appointment and spend the first few seconds confirming what day it actually thinks today is.
Why People Ask This
People ask because changing the first column of a calendar makes an extremely familiar layout feel wrong almost immediately. The same dates and weekdays are still present, yet shifting where the week begins can make people hesitate when reading a schedule they would normally understand without thinking.