First day of the week

In English language countries the week may begin on either Sunday or Monday. Most business and social calendars in the United Kingdom, Canada and Australia mark Monday as the first day of the week, though in South Africa and South America, Monday is considered the first day of the working week. Most calendars in the United States begin on Sunday.

Sunday was the first day of the astrological week, the Hebrew week, and in the Ecclesiastical Latin week of the first millennium.

In Jewish and Christian tradition, the first day of the seven day week is Sunday. According to the Bible, God created the Earth in six days, and rested on the seventh day, the Sabbath, i.e. Saturday. This made Sunday the first day of the week, while Saturdays were sanctified for celebration and rest. After the week was adopted in Early Christian Europe, Sunday remained the first day of the week, but also gradually displaced Saturday as the day of celebration and rest, being considered the Lord's Day.

The variation is evident from names of the days in some languages — in Hebrew, Arabic, Greek, Portuguese some days are simply called by their number starting from Sunday, e.g. Monday is called "Second day" etc. In other languages, like Slavic languages, days are also called after their ordinal numbers, but starting from Monday, making Tuesday the "Second day". According to another possible explanation, days from Monday to Friday in Slavic languages aren't numbered by their position within the week, but by their distance from Sunday, especially given that Wednesday is named "The Middle day", which makes it a true statement only if Sunday is the first day of the week.

Through common usage in most of Europe for business purposes today, in South America, and in parts of Asia, Monday is considered to be the first day of the week and is literally named as such in languages such as Mandarin (xingqiyi) and Lithuanian (pirmadienis). The ISO prescribes Monday as the first day of the week with ISO-8601 for software date formats.

In the Julian and Gregorian calendars, a day extends from midnight to midnight. However, in the Islamic and Hebrew calendars the days extend from sunset to sunset. Thus, the first day of the Islamic calendar, yaum al-ahad, starts on Saturday after sunset and extends to sunset on Sunday. The Jewish sabbath also starts at sunset on Friday and extends into Saturday.