Q. Why is it a mortal sin to miss mass on Sundays?
A. Christ said, “If you want to enter into life, keep the commandments.” Now one of these commandments is,
“Remember the sabbath day and keep it holy.”
Thus God demands the sanctification of one day in seven in a special way. His very use of the word “Remember” implies a grave obligation not to forget or omit this duty. It is a mortal sin to disobey God in this matter. But how are we Christians to observe this commandment? Who is to tell us? Our Lord says,
“If a man will not hear the Church, let him be as the heathen.“Mt. 18:17
We must, then, hear the Church. Now the Catholic Church tells us that the central factor in the religion of Christ is the Mass, and that the chief thing in the sanctification of Sunday is to be present at the offering of that Sacrifice to God. This obliges under pain of mortal sin, unless sickness or other grave difficulties prevent such assistance at Mass. Remember that men are not only individual beings. They are also social beings. Therefore, they are obliged to worship God in their individual capacity and collectively as well. God has always demanded public worship and from the earliest Apostolic times Christians met regularly for religious exercises in common. Radio RepliesVol. 1: #1168
USA : HOLY DAYS OF OBLIGATION
The days to be observed as holy days of obligation in the Latin Rite dioceses of the United States of America, in conformity with canon 1246, are as follows:
January 1, the solemnity of Mary, Mother of God; (Except in Southern California. Why? I don’t know) Thursday of the Sixth Week of Easter, the solemnity of the Ascension;
August 15, the solemnity of the Assumption of the Blessed Virgin Mary;
November 1, the solemnity of All Saints;
December 8, the solemnity of the Immaculate Conception;
December 25, the solemnity of the Nativity of Our Lord Jesus Christ.
Whenever January 1, the solemnity of Mary, Mother of God, or August 15, the solemnity of the Assumption, or November 1, the solemnity of All Saints, falls on a Saturday or on a Monday, the precept to attend Mass is abrogated.
MORTAL SIN=Grave Sin + Knowledge+ Free Choice CCC
2041 The precepts of the Church are set in the context of a moral life bound to and nourished by liturgical life. The obligatory character of these positive laws decreed by the pastoral authorities is meant to guarantee to the faithful the very necessary minimum in the spirit of prayer and moral effort, in the growth in love of God and neighbor:
2042 The first precept (“You shall attend Mass on Sundays and holy days of obligation and rest from servile labor”) requires the faithful to sanctify the day commemorating the Resurrection of the Lord as well as the principal liturgical feasts honoring the mysteries of the Lord, the Blessed Virgin Mary, and the saints; in the first place, by participating in the Eucharistic celebration, in which the Christian community is gathered, and by resting from those works and activities which could impede such a sanctification of these days.82 The second precept (“You shall confess your sins at least once a year”) ensures preparation for the Eucharist by the reception of the sacrament of reconciliation, which continues Baptism’s work of conversion and forgiveness.83
The third precept (“You shall receive the sacrament of the Eucharist at least during the Easter season”) guarantees as a minimum the reception of the Lord’s Body and Blood in connection with the Paschal feasts, the origin and center of the Christian liturgy.84
2043 The fourth precept (“You shall observe the days of fasting and abstinence established by the Church”) ensures the times of ascesis and penance which prepare us for the liturgical feasts and help us acquire mastery over our instincts and freedom of heart.85 The fifth precept (“You shall help to provide for the needs of the Church”) means that the faithful are obliged to assist with the material needs of the Church, each according to his own ability.86
The faithful also have the duty of providing for the material needs of the Church, each according to his own abilities.87
You can view the Precepts of the church in the CCC