Which Day Does the Week Start?

No single answer

The first day of the week is a convention, not a fact of nature. The seven-day week is shared across the world, but the choice of where to put the boundary is not. Standards and countries have made different choices, and many calendars let the user choose.

The international standard ISO 8601 says that a week starts on Monday and ends on Sunday. Most of Europe follows it in everyday use, and so do many countries in other regions. In the United States, Canada and some other countries, calendars traditionally start the week on Sunday. In some countries in the Middle East, the week starts on Saturday or on Sunday, depending on the country and the context.

Why ISO uses Monday

ISO 8601 defines the week as Monday to Sunday, and this has practical consequences that businesses like. The five working days of a typical week form one block at the start, and the weekend, Saturday and Sunday, sits together at the end. A week is therefore a natural unit of work, which is how companies and administrations use week numbers.

The choice also fits the week rule. A week that starts on Monday has Thursday in the middle: it is three days after Monday and three days before Sunday. That makes the Thursday a fair marker for deciding which year the week belongs to, and it is the reason the ISO rule is written around the first Thursday.

Sunday as the first day

In the United States and in Canada, standard wall calendars show Sunday in the first column. Sunday-first calendars have a long tradition in regions with Jewish and Christian heritage, where the week is counted from the day of rest or worship. Software such as Excel uses Sunday as the default first day when the system region is the United States, and many phone calendars follow the region setting.

Several other countries also treat Sunday as the first day, including Japan in many calendar designs, Brazil and Israel, although practice varies and many people in those countries also work with Monday-first planners. The point is that you cannot infer the first day from the country alone. Check the setting in the tool you use.

Saturday and other cases

In some countries in the Middle East and North Africa, the week starts on Saturday. Where the weekend is Friday and Saturday, as in Saudi Arabia and Israel, a work week often runs from Sunday to Thursday, and Sunday behaves like a Monday does in Europe. The United Arab Emirates moved its federal weekend to Saturday and Sunday on 1 January 2022, with a short working day on Friday.

These differences affect more than the look of a calendar. They decide where a week boundary falls, which Sunday or Monday is the start of a pay period, and how many hours count towards a weekly limit. A shift plan for a business with international teams needs a written definition of the first day.

How the first day changes week numbers

The choice of the first day changes the number of dates in the first and last week and can shift the week number of any date by one. With ISO, the week with the first Thursday is week 1. With Sunday start and the rule that the week containing 1 January is week 1, the first week can be as short as one day. With a Monday start and the same rule, the first week can also be short.

The calculator shows the effect directly. For Sunday 25 October 2026, the ISO week is 43, since the week runs Monday 19 to Sunday 25 October. The US number is 44, since Sunday starts a new week. The Monday-start variant gives 43. Looking at all three numbers for the same date is the best way to see how much depends on one convention.

Settings in everyday tools

Most tools let you choose the first day of the week. In calendar apps it is usually in the settings under a name such as "start week on". In spreadsheets, functions such as WEEKDAY and WEEKNUM take an argument that selects the first day. In programming languages and databases, the default differs: some start the week on Sunday and some on Monday, so it is worth checking the documentation before relying on it.

If you move a spreadsheet between two computers with different regional settings, check the results again. A formula that depends on the system default can show different numbers on each machine.

What to do in practice

In personal use, set the first day to what you are used to and keep it. In shared documents, state it. A line such as "weeks run Monday to Sunday, numbered by ISO 8601" removes most of the confusion.

In software, store dates in the ISO form, which is independent of the first day, and calculate week numbers and week starts with a function that takes the rule as a parameter. Hard-coding Sunday or Monday into the logic is a common source of defects in apps that are used in several countries.

The main result of this calculator follows ISO 8601: Monday to Sunday, week 1 with the first Thursday. The extra rows give the Sunday-start and the Monday-start numbers for comparison, so you can match whichever system your partner uses.