Please Urgently Help!!! tell me why Sunday is considered the Sabbath day and not Saturday..I just attended...?

The reason 7th Day Adventists choose Saturday is because technically if the Jews were the chosen people and God told them to regard the Sabbath as a holy day and they observe the Sabbath on Saturday, then they too should say Saturday. I don't know why Christians picked Sunday unless it was to differentiate from the Jews in some manner.