I think there is a distinction between what people consider the first day of the week and what is printed on a calendar.
I don't honestly care much what people consider the beginning of the week, I try to avoid ambiguity in planning as much as I can anyways for other reasons (this Sunday vs next Sunday for eg.) but I have been tripped up by an unexpected Monday-first calendar in a webapp.
If someone says "let's meet the 3rd week of April" and you say "ok, let's do Sunday", do you really mean the Sunday in the 4th calendar row?