@Truck Oh wow, thanks! I didn't realize there was that much variance, but I guess I should have suspected. I've added another poll to add Friday/Saturday at least.
I've lived in the US my whole life, where our calendars usually start on Sunday, but I've always considered Monday to be the first day of the week, and set my calendars to do so when I can. I kinda thought the calendar was some weird historical quirk or something and most in the US thought the same as me, but ran into confusion the other day over it and realized maybe not!