Which Day is the Sabbath of the New Testament?

Which day did Christ and the apostles observe? Which day did Paul teach Gentile converts to observe? How did the day become changed from the seventh to the first day of the week? WE have been reared in a Sunday-observing world. Naturally, we have taken Sunday observance for granted. Naturally, the idea of a different day as the true Sabbath day strikes us as fanatical and absurd.