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It is important to remember the Church does not consider Sunday to be the Sabbath. As the Catechism notes:
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 (2175, emphasis added).
The sabbath, which represented the completion of the first creation, has been replaced by Sunday which recalls the new creation inaugurated by the Resurrection of Christ. (2190, emphasis added).
Christians are freed from the ritual laws of the Old Testament (but not its moral ones). Thus the ritual of observing the Sabbath is no longer binding upon Christians. However, Christians still owe a moral duty to worship God (CCC 2176) and therefore do so on the day of the Resurrection. This is a Christian practice that dates back to the original apostles (see Acts 20:7).