The Catechism in Questions & Answers

318. What is the significance of the Third Commandment in the Old Testament?

God rested on the seventh day of the creation and hallowed it. The day of rest has been given to us as a holiday in which we are to thank God for His work of creation and honour Him.
Already before the law was given on Sinai, God had designated the Sabbath as the day that was to be kept holy. During the journey of the people of Israel through the desert, Moses proclaimed: “This is what the Lord has said: ‘Tomorrow is a Sabbath rest, a holy Sabbath to the Lord” (Exodus 16: 23).
On the Sabbath, the people of Israel were to rest from their work and turn to God without distraction. The Sabbath served to praise the Creator and commemorate the liberation of Israel from captivity in Egypt. Those who honoured the Sabbath and avoided personal business and idle talk (cf. Isaiah 58: 13-14) were promised blessing.