The Bible refers to Saturday as being the Sabbath. How did the Catholic Church arrive at Sunday as the Sabbath, and are we committing a mortal sin by not obeying this Commandment and keeping Saturday holy?
The obligation to set aside time for worship is part of the natural law that is binding on everyone. Not all of the laws found in the Old Testament are part of the natural law, and those which are not part of the natural law—e.g., the obligation to worship on Saturday—were only ever binding on the Jews. Christians are not bound by them. Since Jesus was resurrected from the dead on Sunday, it makes sense for Christians to set aside Sunday for worship. The earliest Christians did this, and Catholics continue to do so still today.