Monday is the first day of the week, according to the international standard for the representation of dates and times ISO 8601.
However, in the United States and Canada, Sunday is considered to be the start of the week.
This is because of religious reasons. For those of Christian and Jewish faith, Sunday is the most important day of the week. Not only is it the day when God created Earth, it also comes after the day of rest, that gets us ready for the week. In Catholicisim, it also states in the Bible that the church is meant to assemble on the first day of the week, which would be Sunday mass.
In the United States, Monday is considered as the first day of the work week, for logistic purposes such as companies paying their employees.
Origins of Sunday as the first day of the Week
Sunday has always been regarded as the first day of the week for religious reasons. Beginning in ancient times, in Egypt, when Sunday was set aside as the most important day of the week, and thus the one that was at the start. The "day of the sun" was observed in honor of the Sun-god, Ra, the chief of all astral bodies, making Sunday the first of all days.
In the Bible, God rested on the seventh day, or the Sabbath (Saturday), after creating the Earth:
"Then God blessed the seventh day and made it holy, because on it he rested from all the work of creating that he had done. (Genesis 2:3)"
In the Jewish faith, this places Sunday as the first day of the week, in accordance with the creation tale, as it comes after the Sabbath.
For Christians, since Pope Constantine, Sunday is the day of worship and rest. It is also the day when Jesus resurrected, and thus the most sacred day in the Christian faith. This makes Sunday the first day of the week, as it is the most important when they must congregate together in church.
"On the first day of the week, when we were gathered together to break bread, Paul began talking to them, intending to leave the next day, and he prolonged his message until midnight.(Acts 20:7)"
Monday as the first day of the Week
With the arrival of industrialization, people began measuring their days according to the five-day work week, which begins on a Monday. In 1878, the term weekend was first recorded as being "the period between the close of one working or business or school week and the beginning of the next", referring to Saturday and Sunday, and placing Sunday at the end of the week.
This was mostly done in European countries, and is likely what the ISO 8601, first published in 1988, was based on, to create a standard of the representation of dates and time. So, most countries in Europe consider Monday to be the first day of the week.
In the United States, Sunday is still considered the first day of the week, while Monday is the first day of the working week.
The First day of the Week in the Gregorian Calendar
The Gregorian calendar is the most used calendar around the world today, including in the United States.
This calendar follows the international standard for Representation of dates and times, which was established in 2004.
According to the ISO 8601:2004 the Gregorian calendar has 52 or 53 7-day weeks, and it is the standard to start the week on Monday. This was done in an attempt to standardize the measures of dates and times for everyone. However, countries like the United States and Canada still observe their own date representation formats, although they do use the Gregorian Calendar.