The law found in the Old Testament (including the Ten Commandments) is known as the Old Law. The Old Law was revealed to the Israelites and, as given, was binding only on them. Jesus came to “fulfill” the Old Law (Mt 5:17) and establish the New Law. The *Catechism of the Catholic Church * (CCC) explains that the Old Law was “holy, spiritual, and good, yet still imperfect” (CCC 1963), while the New Law “’fulfills’, refines, surpasses, and leads the Old Law to its perfection” (CCC 1967).
Common to both the Old Law and the New Law is that part of the law known as natural moral law. “Natural moral law, present in the heart of each man and established by reason, is universal in its precepts and its authority extends to all men” (CCC 1956). The New Law retains all of the natural moral law contained in the Old Law while freeing us from the ritualistic aspects of it. The catechism explains that the New Law is called a *law of freedom * because "it sets us free from the ritual and juridical observances of the Old Law” (CCC 1972).
The Old Law commandment to observe the Sabbath is a good example of this. It contains both natural moral law (rest and worship) and ritual observance (on Saturday). So, while we are bound to observe a day of rest and worship, we are not bound to do it on Saturday. For Christians, Sunday’s ceremonial observance replaces that of the Sabbath. The catechism explains that “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 [Law] taking up its rhythm and spirit in the weekly celebration of the Creator and Redeemer of his people” (CCC 2177).
Why Sunday? “Jesus rose from the dead ‘on the first day of the week.’ 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, 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… 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” (CCC 2174).