1. The second commandment
The second commandment of God's Law is You shall not take the name of the Lord your God in vain. This commandment “prescribes respect for the Lord's name" ( Catechism 2142), and commands us to honor the name of God. The Lord's name is not to be pronounced “except to bless, praise, and glorify it" (Catechism 2143).
1.1 The name of God
“A name expresses a person's essence and identity and the meaning of this person's life. God has a name; he is not an anonymous force" ( Catechism 203). However, God cannot be grasped through human concepts; no idea can represent him and no name can exhaustively express the divine essence. God is “Holy," which means that he is absolutely superior, that he is above any creature, that he is transcendent.
Nevertheless, so that we can invoke him and speak to him, in the Old Testament he “revealed himself progressively and under different names to his people" ( Catechism 204). The name he revealed to Moses indicates that God is Being in essence, the fullness of being and of every perfection. “God said to Moses, 'I Am who I Am.' And he said, 'Say this to the people of Israel: I Am [Yahweh: He is ] has sent me to you'…' This is my name for ever" ( Ex 3:13-15; (cf. Catechism 213). Out of respect for God's holiness, the people of Israel never pronounced this name, replacing it rather by the title “Lord" ( Adonai in Hebrew, Kyrios in Greek) (cf. Catechism 209). There are other names for God in the Old Testament: Elohim , the majestic plural of plenitude or grandeur, and El-Saddai , which means mighty, omnipotent.
In the New Testament, God makes known the mystery of his intimate Trinitarian life: a single God in three Persons—Father, Son and Holy Spirit. Christ teaches us to call God “Father" (Mt 6:9), “Abba," which is the familiar way of saying “father" in Hebrew (cf. Rom 8:15). God is the Father of Christ and our Father, but not in the same way: Jesus is the Only-begotten Son and we are adopted children. But we truly are children (cf. 1 Jn 3:1), brothers and sisters of Jesus Christ (cf. Rom 8:29), because the Holy Spirit has been sent into our hearts and we participate in the divine nature (cf. Gal 4:6; 2 Pet 1:4). We are children of God in Christ. As a consequence we can address God as “Father." St. Josemaria wrote: “God is a Father who is full of tenderness, of infinite love. Call him 'Father ' many times a day and tell him—alone, in your heart—that you love him, that you adore him, that you feel proud and strong because you are his son." 
1.2 Honoring God's name
In the Our Father we pray: “Hallowed be thy name." The term “hallowed" means in this context “to recognize as holy, to treat in a holy way" (Catechism 2807). This is what we do when we adore, praise or give thanks to God. But “hallowed be thy name" is also one of the petitions of the Our Father: in saying it we ask that his name be hallowed through us, that is, that we give him glory by our life and that others may glorify him (cf. Mt 5:16). “The sanctification of his name among the nations depends inseparably on our life and our prayer" (Catechism 2814).
Respecting the name of God also calls for respecting the name of the Blessed Virgin Mary and of the saints, and respecting holy things where God is present in one way or another, especially the Holy Eucharist, the real Presence among us of Jesus Christ, Second Person of the Holy Trinity.
The second commandment forbids the improper use of God's name (cf. Catechism 2146), and especially blasphemy , which “consists in uttering against God—inwardly or outwardly—words of hatred, reproach, or defiance… It is also blasphemous to make use of God's name to cover up criminal practices, to reduce peoples to servitude, to torture persons or put them to death … Blasphemy … is in itself a grave sin" ( Catechism 2148).
It also forbids false oaths (cf. Catechism 2850). Taking an oath is to take God as a witness for what is affirmed (for example, to guarantee a promise or a testimony, to prove the innocence of a person unjustly accused or suspected, or to put an end to litigation and controversy, etc.). There are circumstances when an oath is lawful if it is taken in truth and justice, and if it is necessary, as may be the case in a trial or on assuming office (cf. Catechism 2154). Otherwise, the Lord teaches us not to swear: Let what you say be simply 'Yes' or 'No' (Mt 5:37; cf. Jas 5:12; Catechism 2153).
1.3 The name of a Christian
Man “is the only creature on earth that God has willed for its own sake."  He is not “something" but “someone," a person. “He alone is called to share, by knowledge and love, in God's own life. It was for this end that he was created, and this is the fundamental reason for his dignity" (Catechism 356). In baptism, on being made a child of God, each person receives a name that represents his or her unrepeatable singularity before God and before others (cf. Catechism 2156, 2158). Baptism is also called “christening." A “Christian," a follower of Jesus Christ, is the proper name of every baptized person, who has received the call to identify himself with the Lord: it was in Antioch that the disciples were first called 'Christians' (Acts 11:26).
God calls each person by name (cf. 1 Sam 3:4-10; Is 43:1; Jn 10:3; Acts 9:4). He loves each one personally. Christ, says St Paul, loved me and gave himself up for me (Gal 2:20). He expects from everyone a loving response: you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength (Mk 12:30) Nobody else can give God this response of love in our place. St Josemaria encourages us to meditate “calmly that divine admonition which fills the soul with disquiet and which at the same time tastes as sweet as honey from the comb: redemi te, et vocavi te, nomine tuo: meus es tu ( Is 43:1); I have redeemed you and called you by your name: you are mine! Let us not steal from God what belongs to him. A God who has loved us to the point of dying for us, who has chosen us from all eternity, before the creation of the world, so that we may be holy in his presence (cf. Eph 1:4)." 
2. The third commandment of the Decalogue
The third commandment of the Decalogue is: Keep feast days holy.
2.1 Sunday or the Lord's Day
The Bible recounts the work of creation in six “days." At the end God saw everything that he had made, and behold, it was very good … So God blessed the seventh day and hallowed it, because on it God rested from all his work which he had done in creation (Gen 1:31; 2:3). In the Old Testament, God decreed that the seventh day of the week should be holy, a day set off and distinct from the others. Man, who was called to participate in the creative power of God, perfecting the world through his work, was also to cease working on the seventh day in order to dedicate it to divine worship and to rest.
Before the coming of Jesus Christ, the seventh day was the Sabbath. In the New Testament it is Sunday, Dies Domini , the day of the Lord, because it is the day of the Lord's Resurrection. The Sabbath represented the end of creation; Sunday represents the beginning of the “new creation," which took place with Christ's resurrection (cf. Catechism 2174).
2.2 Participating in Mass on Sunday
Since the sacrifice of the Eucharist is the “fountain and the summit of the life of the Church,"  Sunday is kept holy mainly by participating in holy Mass. The Church expressed the third commandment of the Decalogue concretely in the following decree: “On Sundays and other holy days of obligation the faithful are bound to participate in the Mass" (CIC, can. 1247; Catechism 2180). Outside of Sundays, the main days of obligation are: “Christmas, the Epiphany, the Ascension, the Body and Blood of Christ, Mary the Mother of God, her Immaculate Conception, her Assumption, Saint Joseph, the Apostles Saints Peter and Paul, and lastly All Saints" (CIC, can. 1246; Catechism 2177). “The precept of participating in the Mass is satisfied by assistance at a Mass which is celebrated anywhere in a Catholic rite either on the holy day or on the evening of the preceding day" (CIC, can. 1248; Catechism 2180).
“… the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor (cf. CIC, can. 1245). Those who deliberately fail in this obligation commit a grave sin" ( Catechism 2181).
2.3 Sunday, day of rest
“Just as God 'rested on the seventh day from all his work which he had done' (Gen 2:2), human life has a rhythm of work and rest. The institution of the Lord's Day helps everyone enjoy adequate rest and leisure to cultivate their familial, cultural, social, and religious lives" ( Catechism 2184). On Sundays and other holy days of obligation, the faithful are to refrain “from work and activities that hinder the worship owed to God, the joy proper to the Lord's Day, and the appropriate relaxation of mind and body" (CIC, can. 1247). This constitutes a serious obligation, as is that of sanctifying holy days. However, a more important duty of justice or charity can excuse one from the obligation of Sunday rest.
“In respecting religious liberty and the common good of all, Christians should seek recognition of Sundays and the Church's holy days as legal holidays. They have to give everyone a public example of prayer, respect, and joy and defend their traditions as a precious contribution to the spiritual life of society" (Catechism 2188). “Every Christian should avoid making unnecessary demands on others that would hinder them from observing the Lord's Day" (Catechism 2187).
2.4 Public worship and the civil right to religious freedom
At present in certain countries, a fairly widespread “laicist" attitude tends to view religion as a private matter which should have no public and social manifestations. Christian doctrine, on the contrary, teaches that people must “be able freely to profess their religion in private and in public."  In fact, natural moral law, inscribed in man's heart, prescribes “giving God exterior, visible, public worship"  (cf. Catechism 2176). Of course worshiping God is first of all a private act, but people need to be able to manifest it exteriorly because the human spirit “needs to use material things as signs by means of which it is prompted to carry out the spiritual acts that unite it to God." 
People need to be free to profess religion not only exteriorly, but also socially, that is, with others, because “the social nature of man itself requires that … he should profess his religion in community."  “Injury therefore is done to the human person … if the free exercise of religion is denied in society when the just requirements of public order do not so require … Government, therefore, ought indeed to take into account the religious life of the people and show it favor, since the function of government is to make provision for the common welfare." 
There is a social and civil right to freedom in religious matters. Society and the government cannot hinder anyone from acting in this area in accord with the dictates of their conscience, whether in private or in public, while respecting just limits derived from the requirements of the common good, such as public order and public morality  (cf. Catechism 2109). Every person is obliged in conscience to seek the true religion and to adhere to it. In this search they can receive the help of others (and Christian faithful, moreover, have the duty to provide this help in their apostolate), but nobody should be forced either to act against their convictions or prevented from acting in accord with their conscience in religious matters. Embracing the faith and practicing it always have to be free acts (cf. Catechism 2104-2106).
“Your task as a Christian citizen is to help see Christ's love and freedom preside over all aspects of modern life: culture and the economy, work and rest, family life and social relations." 
Second commandment: Catechism of the Catholic Church 203-213; 2142-2195.
Third commandment: Catechism of the Catholic Church 2168-2188; John Paul II, Apostolic letter Dies Domini , 31 May 1998.
Benedict XVI-Joseph Ratzinger, Jesus of Nazareth Part I , Ignatius Press, (ch. 5, para. 2).
St Josemaria, “Getting to know God," in Friends of God , 142-153.
[ 1] Friends of God , 150.
 Vatican Council II, Const. Gaudium et Spes , 24.
 St Josemaria, Friends of God , 312.
 Vatican Council II, Const. Sacrosanctum Concilium , 10.
 Vatican Council II, Decl. Dignitatis humanae , 15; Catechism , 2137.
 St Thomas Aquinas, Summa Theologiae , II-II, q. 122, a. 4, c.
 St Thomas Aquinas, Summa Theologiae , II-II, q. 81, a. 7, c.
 Vatican Council II, Decl. Dignitatis humanae , 3.
 St Josemaria, Furrow , 302.