The person originally bringing this up referenced printed calendars always having Monday as the first day of the week, so it seems reasonable to use an observation of printed calendars with Sunday as the start of the week as a rebuttal.
Yes this, except it's not even a rebuttle, I'm just genuinely curious where this might be someone's experience because it's very at odds with my own.
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.
