[tsimionescu]

The USA was founded and its culture heavily influenced by very deeply religious Protestants. While by the time of the founding of the USA, Sunday was decently well established as the seventh day in both Catholicism and Orthodoxy, most protestants were Bible originalists.

I believe that they resurrected the practice of considering Sunday as the first day of the week, based on the Old Testament tradition. Just like New Earth Creationism is in fact a relatively recent phenomenon driven by a renewed interest in literal interpretations of the Bible, that had long since been superseded in Catholic scholarship.

[gampleman]

Actually Orthodoxy is arguably more inclined to treat Sunday as the first day of the week as witnessed by the Greek language.

There are also several patristic writings that conceptualise Sunday as the eighth day of creation, i.e. the resurrection perfecting and completing creation (this also suggests that literal seven day creation wasn’t taken that… er… literally back in the day).