By tradition derived from ancient Jews, Saturday is the last day of the week. That convention remains universally standard in the USA, but in modern Europe many people now consider Saturday the sixth (penultimate) day of the week, and Sunday the last. The modern European convention has been formalized by ISO 8601.
In ancient Jewish tradition Saturday is the sabbath. Many languages lack separate words for "Saturday" and "sabbath". Eastern Orthodox churches distinguish between the sabbath (Saturday) and the Lord's day (Sunday). Roman Catholics put so little emphasis on that distinction that many among them follow -- at least in colloquial language -- the Protestant practice of calling Sunday the sabbath.
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