The Catechism devotes an entire article to this subject.
2173 The Gospel reports many incidents when Jesus was accused of violating the sabbath law. But Jesus never fails to respect the holiness of this day. He gives this law its authentic and authoritative interpretation: “The sabbath was made for man, not man for the sabbath.” With compassion, Christ declares the sabbath for doing good rather than harm, for saving life rather than killing. The sabbath is the day of the Lord of mercies and a day to honor God. “The Son of Man is lord even of the sabbath.”
2175 Sunday is expressly distinguished from the sabbath which it follows chronologically every week; for Christians its ceremonial observance replaces that of the sabbath. In Christ’s Passover, Sunday fulfills the spiritual truth of the Jewish sabbath and announces man’s eternal rest in God. For worship under the Law prepared for the mystery of Christ, and what was done there prefigured some aspects of Christ:
2176 The celebration of Sunday observes the moral commandment inscribed by nature in the human heart to render to God an outward, visible, public, and regular worship “as a sign of his universal beneficence to all.” Sunday worship fulfills the moral command of the Old Covenant, taking up its rhythm and spirit in the weekly celebration of the Creator and Redeemer of his people.
2184 Just as God “rested on the seventh day from all his work which he had done,” 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.
2185 On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body. Family needs or important social service can legitimately excuse from the obligation of Sunday rest. The faithful should see to it that legitimate excuses do not lead to habits prejudicial to religion, family life, and health.
The charity of truth seeks holy leisure- the necessity of charity accepts just work.
2186 Those Christians who have leisure should be mindful of their brethren who have the same needs and the same rights, yet cannot rest from work because of poverty and misery. Sunday is traditionally consecrated by Christian piety to good works and humble service of the sick, the infirm, and the elderly. Christians will also sanctify Sunday by devoting time and care to their families and relatives, often difficult to do on other days of the week. Sunday is a time for reflection, silence, cultivation of the mind, and meditation which furthers the growth of the Christian interior life.