The Third Commandment is: Remember to keep holy the Lord's Day.
As the book of Genesis relates, God created the world in six days, and on the seventh day God rested. So He has commanded us to rest on the seventh day from work, and to give it to the service of God. The Cure of Ars said: "God made seven days in the week. He could have kept them all. He gave you six and asks one for Himself. What right have you to take what does not belong to you?"

The Catechism tells us that we are commanded by the Third commandment to give special worship to God at appointed times, and we sin  against it by willfully failing to sanctify Sunday and Holy Days of Obligation as the Church prescribes. The Church prescribes that we assist at Holy Mass; and she forbids us to engage in unnecessary bodily work on Sundays and Holy Days of Obligation. Our Divine Lord says: "The Sabbath was made for man, not man for the Sabbath." (Mark 2:27). The Sabbath was made to provide two of man's most essential needs, firstly, periodic rest from bodily toil, secondly greater opportunity to attend to his soul and to God. (Of course, necessary work is always allowed.) 

The reason for this Commandment.  (St. Thomas Aquinas)
To increase our love, we are commanded to keep holy the Sabbath, for the corruptible body is a load upon the soul. And man always tends downwards toward earthly things unless he takes means to raise himself above them. It is indeed necessary to have a certain time for this. In fact, some do this continually; "I will bless the Lord at all times; His praise shall be ever in my mouth," (Ps. 33:1). And again; "Pray without ceasing." (1 Thess. 5:17). These shall enjoy the everlasting Sabbath. And some, in order to avoid being entirely apart from God, find it necessary to have a fixed day (for worship,) lest they become too lukewarm in their love of God: "If you call the Sabbath delightful....then shalt thou be delighted in the Lord." (Isa 58:13-14.) And accordingly this day is not set aside for the sole exercise of games, but to praise and pray to the Lord God. Wherefore St. Augustine says that it is a lesser evil to plow than to play games on this day.

The meaning of holy: We ought to know that God did not simply say to "keep" the Sabbath, but to remember to " keep it holy," The word holy may be taken in two ways"     a) Holy signifies "pure." Sometimes holy (or sanctified) is the same as "pure"; "You are washed; you are sanctified". (1 Cor. 6:11)
(i.e., "you are made holy"). b) Holy signifies "consecrated". Then again at times holy is said of a thing consecrated to the worship of God, as for instance, a place, a season, vestments, and the holy vessels.

Offer sacrifice
. In the Book of Numbers, (Num 28) God commands that every day one lamb be offered in the morning and one in the evening, but on the Sabbath day the number should be doubled. This shows that on the Sabbath we should offer sacrifice to God from all that we possess; "All things are Thine, and we have given Thee what we received from Thy hand." (Rom. 12:1). We should offer our soul to God, being sorry for our sins, and we should pray for His blessings. Feast days were instituted for that spiritual joy which is the effect of prayer. Therefore on such days our prayers should be multiplied. We should sacrifice our possessions by giving alms: "And do not forget to do good and to impart, for by such sacrifice God's favor is obtained." (Heb. 13:16). Rest in divine contemplation. This is for the quiet of the soul, for just as the tired body desires rest, so also does the soul. But the soul's proper rest is in God: "Be Thou unto me a God, a protector, and a house of refuge." (Ps. 30:3.) 

Rests that precede divine contemplation. Before the soul arrives at this rest, three other rests must precede:  1) Rest from the turmoil of sin: "But the wicked are like the raging sea which cannot rest." (Isa. 57:20) 2) Rest from the passions of the flesh: "For the flesh lusteth against the spirit, and the spirit against the flesh." (Gal. 5:17) 3) Rest from the occupations of the world: "Martha, Martha, thou art careful and art troubled about many things." (Luke. 10:41.)  Eternal Rest. And then after all these things, the soul rests peacefully in God. The saints gave up everything to possess this rest, "for it is a pearl of great price which a man having found, hid it, and for joy thereof giveth and selleth all that he hath and buyeth that field." (Matt. 13:45-46.) This rest in truth is eternal life and heavenly joy. "This is my rest for ever and ever; here will I dwell, for I have chosen it." (Ps. 131:14).
And to this rest may the Lord bring us all!

Source: Rev. W. Frean, C.SS.R. Commentary on the Catechism
St. Thomas Aquinas, God's Greatest Gifts.