Heard an interesting bit on the radio about this. They said it depends on your culture and religion. Some places, like Egypt, still use a Saturday to Friday week. My old calendar from the bank started weeks on Monday, which always looked wrong to me. I guess it's just what you grow up with.
Yeah, it's wild how many little things like that are just local. Like which floor is the "first floor" in a building. Or how you write the date. Always trips me up when I travel.