"Monday is the first day of the week according to the international standard ISO 8601, but in the US, Canada, and Japan, it's counted as the second day of the week." - https://www.timeanddate.com/calendar/days/monday.html
I've always thought of Monday as the first day of the week. It's the day that starts the workweek (and school week) and Sunday is part of the weekend.
Sunday is actually the first day of the week (i.e. column index = 0) in almost every calendar I've used. Perhaps this varies by culture/locale?