I had always taken it as an article of faith (not religious faith) that the week started on a Sunday and ended on a Saturday. Mainly because when I was young my day told me so.
Where I work, operations have a week Monday to Sunday, HR and finance have a week Sunday to Saturday. We use several IT tools, the US ones give the week number starting on a Monday, the european ones Sunday.
So I have no idea. I will continue to consider the week starting on a Sunday simply because I have always done so. I can find no "official" definition - I'm delighted if one exists