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. On it you shall not do any work, you, or your son, or your daughter, your male servant, or your female servant, or your livestock, or the sojourner who is within your gates. For in six days the Lord made heaven and earth, the sea, and all that is in them, and rested on the seventh day. Therefore the Lord blessed the Sabbath day and made it holy.
Catechism of the Catholic Church:
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.
Westminster Larger Catechism:
Question 116: What is required in the fourth commandment?
Answer: The fourth commandment requires of all men the sanctifying or keeping holy to God such set times as he has appointed in his Word, expressly one whole day in seven; which was the seventh from the beginning of the world to the resurrection of Christ, and the first day of the week ever since, and so to continue to the end of the world; which is the Christian sabbath, and in the New Testament called the Lord’s day.
Question 117: How is the sabbath or the Lord’s day to be sanctified?
Answer: The sabbath or Lord’s day is to be sanctified by an holy resting all the day, not only from such works as are at all times sinful, but even from such worldly employments and recreations as are on other days lawful; and making it our delight to spend the whole time (except so much of it as is to betaken up in works of necessity and mercy) in the public and private exercises of God’s worship: and, to that end, we are to prepare our hearts, and with such foresight, diligence, and moderation, to dispose and seasonably dispatch our worldly business, that we may be the more free and fit for the duties of that day.
Yeah, that was yesterday. :-)