In the most countries, the first day of the week is Monday or Sunday. But there are also countries where week starts on Saturday.
Monday is the first day of the week almost in every country in Europe (Germany, France, United Kingdom, Italy, Spain), but also in Australia, Russia, Turkey, New Zealand etc.
Sunday is the first day of the week in North America (United States, Canada, Mexico), Brazil, India, China, Japan etc.
Saturday is the first day of the week in some North African countries (Egypt, Lybia, Algeria) and Middle East countries (Iraq, Iran, Qatar), Afghanistan etc.