Show me that they believe this.
I posted a link, but apparently it's not allowed since that whole post is now gone.
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:
Every Sunday
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
II. THE PRECEPTS OF THE CHURCH These are the bare minimum that a person must do to be in good standing in the Faith of Jesus Christ.
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.