For Jews there is no question about it, the seventh day is Saturday, the Shabat. But what even some of the Jews don't realize is that the Jewish day begins on the evening of the previous secular day. This is why the Shabat begins on the secular Friday evening. If you want to know why the Jewish day begins in the evening- read Genesis 1:
5 And God called the light Day, and the darkness He called Night. And there was evening and there was morning, one day.
Another thing about Genesis 1: The Christians who divided the bible into chapters deliberately stopped the chapter at the sixth day. Genesis 2 then begins with the last 3 logical verses about the seventh day, and then continues with the second version of the creation story.
As an Israeli Jew, I was amazed to discover that people in Europe and USA think that the week begins on Monday. Also in some dialects of German for example, they call Saturday "Sonnabend" although there is a word for Saturday- "Samstag". I don't know what Sonnabend means exactly- eve of sunday (Sonntag), eve of the sun (die Sonne) ? maybe one of our German members knows. I think in some other languages they actually named Sunday as "Samstag" and also in English some refer to Sunday as the Sabbath.