[caf]

I believe in my entire life I've encountered exactly one situation in which the day that is defined as the first day of the week actually made a difference to anything: my kids' swim school schedule, where Week N of the term runs from Sunday through to the following Saturday inclusive.

Since the working week (and the school week) here starts on Monday regardless of whether Sunday or Monday is regarded as the first day of the week, it seems to have always been a distinction without a difference to me.