Mark:
the CCC lists the following for venial sin:
1862 One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
vatican.va/archive/ENG0015/__P6C.HTM
Our obligations are part of moral law, as evidenced by the following section of the CCC:
THE MORAL LAW
1950 The moral law is the work of divine Wisdom. Its biblical meaning can be defined as fatherly instruction, God’s pedagogy. It prescribes for man the ways, the rules of conduct that lead to the promised beatitude; it proscribes the ways of evil which turn him away from God and his love. It is at once firm in its precepts and, in its promises, worthy of love.
1951 Law is a rule of conduct enacted by competent authority for the sake of the common good. the moral law presupposes the rational order, established among creatures for their good and to serve their final end, by the power, wisdom, and goodness of the Creator. All law finds its first and ultimate truth in the eternal law. Law is declared and established by reason as a participation in the providence of the living God, Creator and Redeemer of all. "Such an ordinance of reason is what one calls law."2
Alone among all animate beings, man can boast of having been counted worthy to receive a law from God: as an animal endowed with reason, capable of understanding and discernment, he is to govern his conduct by using his freedom and reason, in obedience to the One who has entrusted everything to him.3
1952
There are different expressions of the moral law, all of them interrelated: eternal law - the source, in God, of all law; natural law; revealed law, comprising the Old Law and the New Law, or Law of the Gospel;** finally, civil and ecclesiastical laws**.
1953 The moral law finds its fullness and its unity in Christ. Jesus Christ is in person the way of perfection. He is the end of the law, for only he teaches and bestows the justice of God: "For Christ is the end of the law, that every one who has faith may be justified."4
vatican.va/archive/ENG0015/__P6T.HTM
Bolding mine.
From the Compendium of the CCC:
in the section on the Commandment to “Remember the Sabbath and Keep it Holy.”
453. How does one keep Sunday holy?
2177-2185
2192-2193
Christians keep Sunday and other days of obligation holy by participating in the Eucharist of the Lord and by refraining from those activities which impede the worship of God and disturb the joy proper to the day of the Lord or the necessary relaxation of mind and body. Activities are allowed on the Sabbath which are bound up with family needs or with important social service, provided that they do not lead to habits prejudicial to the holiness of Sunday, to family life and to health.
vatican.va/archive/compendium_ccc/documents/archive_2005_compendium-ccc_en.html#Gods%20Salvation:%20Law%20and%20Grace
Participating in abuses of, or failing to defend, the Liturgy, the public act of worship, is a clear violation of the third commandment, in its own right. All violations of the decalogue are sins.