How does Jesus deal with the Sabbath?
Jesus observes the Sabbath, but at the same time he deals with it very liberally, as one who has complete command over it: "The sabbath was made for man, not man for the sabbath" (Mk 2:27).
The fact that Jesus claims the right to heal on the Sabbath and to interpret the Sabbath laws mercifully poses a dilemma for his Jewish contemporaries: Either Jesus is the Messiah sent by God, which makes him "Lord even of the sabbath" (Mk 2:28), or else he is merely a man, in which case his actions on the Sabbath are a sin against the Law.
Why do Christians replace the Sabbath with Sunday?
Christians replaced the celebration of the Sabbath with the celebration of Sunday because Jesus Christ rose from the dead on a Sunday. The "Lord's Day", however, does include elements of the Sabbath.
The Christian Sunday has three essential elements: (1) It recalls the creation of the world and communicates the festive splendor of God's goodness to the passage of time. (2) It recalls the "eighth day of creation", when the world was made new in Christ (thus a prayer from the Easter Vigil says: "You have wonderfully created man and even more wonderfully restored him."). (3) It includes the theme of rest, not just to sanctify the interruption of work, but to point even now toward man's eternal rest in God. (YOUCAT questions 363-364)
Dig Deeper: CCC section (2173-2176) and other references here.
Part 3: Life in Christ (1691 - 2557)
Section 2: The Ten Commandments (2052 - 2557)
Chapter 1: You Shall Love the Lord Your God with All Your Heart, and with All Your Soul, and with All Your Mind (2083 - 2195)
Article 3: The Third Commandment (2168 - 2195)
Remember the sabbath day, to keep it holy. Six days you shall labor, and do all your work; but the seventh day is a sabbath to the Lord your God; in it you shall not do any work.90
The sabbath was made for man, not man for the sabbath; so the Son of Man is lord even of the sabbath.91 ⇡
I. THE SABBATH DAY ⇡
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.98 He gives this law its authentic and authoritative interpretation: "The sabbath was made for man, not man for the sabbath."99 With compassion, Christ declares the sabbath for doing good rather than harm, for saving life rather than killing.100 The sabbath is the day of the Lord of mercies and a day to honor God.101 "The Son of Man is lord even of the sabbath."102
90.
Ex 20:8-10; cf. Deut 5:12-15.
91.
98.
99.
100.
Cf. Mk 3:4.
101.
102.
II. THE LORD'S DAY ⇡
This is the day which the LORD has made; let us rejoice and be glad in it.103 ⇡
The day of the Resurrection: the new creation ⇡
Jesus rose from the dead "on the first day of the week."104 Because it is the "first day," the day of Christ's Resurrection recalls the first creation. Because it is the "eighth day" following the sabbath,105 it symbolizes the new creation ushered in by Christ's Resurrection. For Christians it has become the first of all days, the first of all feasts, the Lord's Day (he kuriake hemera, dies dominica) Sunday: We all gather on the day of the sun, for it is the first day [after the Jewish sabbath, but also the first day] when God, separating matter from darkness, made the world; and on this same day Jesus Christ our Savior rose from the dead.106
103.
104.
Cf. Mt 28:1; Mk 16:2; Lk 24:1; Jn 20:1.
105.
106.
St. Justin, I Apol. 67:PG 6,429 and 432.
Sunday — fulfillment of the sabbath ⇡
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:107 Those who lived according to the old order of things have come to a new hope, no longer keeping the sabbath, but the Lord's Day, in which our life is blessed by him and by his death.108
107.
Cf. 1 Cor 10:11.
108.
St. Ignatius of Antioch, Ad Magn. 9,1:SCh 10,88.
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."109 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.
109.
St. Thomas Aquinas, STh II-II,122,4.