So, the standard US weekend is Saturday–Sunday, even amongst the Sunday=1 holdouts. I say holdouts, because it helps to understand that this isn’t agreed on even within the US. Not to suggest that the holdouts are a minority, I honestly don’t know but I doubt it. I think, apart from the initial puzzlement of recognizing the inconsistency, most of us don’t give much thought at all to which day is 1. “Weekend” generally is used colloquially to mean whichever two simultaneous days the speaker or audience is off work, insofar as they have two consecutive days off work.

I believe, but I may be wrong, that the inconsistency arises from Sunday being the Christian sabbath. That “starts” the week, but it’s a traditional day off for religious observance. And this tradition goes back well before the 40 hour work week, and the common Monday–Friday work week. Which is to say that “weekend” didn’t originally have connotations about which days were work days, they were typically all work days except for the Christian sabbath. Which as a retcon makes the present colloquial usage even odder, even if it’s (maybe?) more consistent with how other countries/cultures use it.