A Dialogue with God about the Sabbath

Most people who call themselves Christians observe Sunday. However, Jews (and some other nations) observe the Sabbath. Does this day belong only to the Jews or only to ancient Israel? Is Sunday mentioned as the Lord's Day in the New Testament? - Is Saturday still valid? Does it make any difference? If so, what day is the Christian Sabbath? Can it be proven? This is exactly what we want to do in this book, so I encourage you to read it...