The Catholic Church tells us that we have an obligation to go to Mass every Sunday. It is an obligation that most of us fulfill willingly, but many people don't understand why the church requires this of us.
In the Third Commandment, God tells us to "Remember to keep holy the Sabbath day." For the Jews, the Sabbath was Saturday; Christians, however, transferred the Sabbath to Sunday, the day of Jesus Christ's resurrection from the dead. The Church tells us that we have an obligation to fulfill the third Commandment by refraining from unnecessary work on Sunday and by participating in the Mass, our chief form of worship as Christians.
From the earliest days of the Church, Christians have understood that being a Christian isn't a private matter. We are called to be Christians together; while we can and should engage in the private worship of God throughout the week, our primary form of worship is public and communal, which is why Sunday Mass is so important.