The Old Testament Sabbath (and that still celebrated by Jews to this day) is Saturday, but just as Christ influenced every other aspect of life, so did He influence our weekly day of rest and worship dedicated to God. As you’re learning about keeping the Lord’s Day holy this week, you might be interested in these paragraphs from the Catechism:
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 (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.
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:
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.
Catechism of the Catholic Church 2174-2175