We have 2 different calenders. One mentioning Sunday as the first day of the week, the other Monday. Asking in the surrounding it seems that most people are of the opinion that it is Sunday.However I am not a very religious man, I have always learned that Sunday was created as the last day of the week (rest day after week of hard work) It seems weird to me to start the week with a rest day.
What Is the First Day of the Week?
The Bible clearly makes the Sabbath the last day of the week, but does not share how that corresponds to our 7 day week. Yet through extra-biblical sources it is possible to determine that the Sabbath at the time of Christ corresponds to our current 'Saturday.' Therefore it is common Jewish and Christian practice to regard Sunday as the first day of the week (as is also evident from the Portuguese names for the week days). However, the fact that, for example, Russian uses the name "second" for Tuesday, indicates that some nations regard Monday as the first day.
In international standard ISO-8601 the International Organization for Standardization (ISO) has decreed that Monday shall be the first day of the week.
Position during a week
In many countries, including most of Europe, Asia, and South America (except Brazil), Sunday is held to be the last day of the week. In others, including the United States and Canada, it is seen as the first day, a traditional view derived from ancient Jews, Egyptians and the Holy Roman Empire. ISO 8601 defines Monday as the first day of the week, making Sunday the seventh.