Every human society must have power to make laws binding in some way on its subjects. Otherwise it will fail to achieve the purpose for which it came into existence. Even considered as a human society the Catholic Church is the greatest society both in size and numbers that the world has ever seen. She is not only  a human society but she is also divine; because Jesus Christ is the Founder, her Savior and her Head, and she is His Mystical Body. Jesus Christ gave to the rulers of His Church, in the persons of St. Peter and the Apostles, the power to make laws, which would be binding not only on earth, but would be ratified in Heaven, when He said: "Whatsoever you shall bind on earth shall be bound also in Heaven; and whatsoever you shall loose on earth shall be loosed also in Heaven." (Matthew 18:18.)

The Code of Canon Law.
The laws of the Church are contained in five books called the Code of Canon Law. They concern all the members of the Church, bishops, priests and people, religious and lay-people. Those laws which are mentioned in the Catechism are the ones which concern most the general lives of the faithful.
These are:
1. To hear Mass every Sunday and on Holy Days of obligation.
2. To fast and abstain on the days commanded.
3. To confess our sins at least once a year.
4. To receive worthily the Blessed Eucharist each year at Easter, or within the appointed time.
5. To contribute to the support of our pastors and to the upkeep of Catholic Schools and charitable institutions.
6. To observe the Laws of the Church regarding the celebration of the Sacrament of Marriage.
There are some things ordered or forbidden by the Commandments of God, that are good or evil in themselves. For example the love of God is good, blasphemy is evil. Nothing can ever change this.

Disciplinary Laws.
There are other things which can change according to places and circumstances; for example, the manner or time of honoring God, and of fulfilling our duties to wards Him. It is such things as these, for the most part, that are regulated by the Commandments of the Church. So that, as a general rule, they are applications to various times and places of the things commanded by the Decalogue (the Ten Commandments). Hence, in breaking the Commandments of the Church, we also break the Commandments of God.

Sundays and Holy Days:
What has been said is clear in the case of the First Commandment of the Church. In the Old Law the Jews were bound by the Third Commandment of God to keep holy the Saturday called the Sabbath. The Church has changed the Lord's Day from Saturday to Sunday. She also, as God's appointed interpreter of His law, tells us how it is to be kept holy. First we must hear Holy Mass. As human beings we have various kinds of obligations. We have private duties as private individuals, and public duties as members of society, for example, fighting for the right of the life of the unborn. Thus we have national days for public celebration. In like manner God is entitled not only to private worship, which we pay Him when we pray in secret; but also He is entitled to public worship. Thus the Jews offered public worship in the Temple and in the synagogues. In the Mass we fulfill both of these obligations, worshiping God in the secrecy of our hearts; and offering Him public worship at the same time.

If on Sunday we miss any part of the Mass at all, at the beginning or at the end, through our own fault, we are guilty of sin as failing in part of our obligations. If we miss a notable portion of the Mass, through our own fault, we are guilty of mortal sin. Thus those that come after the Offertory has begun are guilty of mortal sin.

Attention is required.
We are bound also to be present not only in body but in mind as well. To do anything that would deliberately distract the mind altogether from the Mass, such as reading a book or paper (other than the Missal) during Mass, would nullify our action and we would not be hearing Mass. Distractions which are in-deliberate should be put away when we become aware of them.

Source: Rev. W. Frean, Commentary on the Catechism