The world has always had multiple calendars in use. The one used in the United States is called the Gregorian Calendar. In an attempted to gain consistency, the International Standards Organization came up with a standard way to express dates and times in 1988. It was published as ISO-8601.
ISO-8601 uses the Gregorian Calendar as the base but, for some reason, the committee chose to start their weeks with Monday. I cannot locate any justification for this choice. It departs from thousands of years of common usage. My guess is that the committee wanted the week to start with the typical first workday.
Regardless of what committees and governments wish to declare or what names they wish to give to various days of the week, it remains that we divide time into weeks because God made the world in six days and rested on the seventh (Genesis 2:2-3).
The seventh day was designated the day of worship under the Law of Moses (Deuteronomy 5:12-14). We call that day "Saturday." The Jews call it Sabbath ("rest"). Even today, those who still follow the Jewish religion continue to worship on Saturday, regardless of the silly rearrangement by ISO-8601.
For Christians, the first day of the week is the day of worship (Acts 20:7; I Corinthians 16:1). That day's name is currently "Sunday." It is the day that follows the Sabbath. Sunday remains the day that Christians gather together to worship God. It remains the first day of the week, regardless of some committee's decision to change it.