The third commandment is “Remember to keep holy the Lord’s day.” This commandment obliges us to assist at Mass on Sundays and to abstain from all unnecessary servile work.
We observe Sunday as a holy day because Our Lord rose from the dead on Easter Sunday, and the Holy Spirit descended to the Apostles on Pentecost Sunday.
Day of rest
The Catechism of the Catholic Church states: “On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body.”
From Monday to Saturday we are mostly occupied with caring for our bodies, hence on Sundays we should in a special manner worship God and care for our souls.
Father John Hardon, SJ, author of The Question and Answer Catholic Catechism, writes, “The Church decreed that Christians observe Sunday as a day of rest because our religion and health require some relaxation at regular times. Moreover, Sunday rest indicates that all have the right and duty to enjoy leisure time allowed by divine and human law. They are to use this leisure to cultivate their social, religious, and family life.”
Servile work is allowed on Sunday when the honour of God, our own need, or that of our neighbour requires it. Hence, work of daily necessity, such as cooking and caring for domestic animals, may be performed on Sunday. Also, servile work necessary for the common good, or to prevent serious financial loss, is permitted on Sunday.
Day of worship
To attend Sunday Mass is a serious obligation for Catholics. Father John Hardon points out that “the causes that might excuse from assisting at Sunday Mass are: physical impossibility, which applies to those who are unable to hear Mass because they are sick, or who have no priest to say Mass for them; moral impossibility, when it would be very difficult to attend Mass, say because of the absolute necessity of fulfilling other grave duties; and the practice of charity, when Mass is sacrificed to remain at the bedside of the sick or give urgent assistance to someone in great need.”
We can sum up the reasons for attending Sunday Mass in four letters: A-R-T-S: for Adoration, Reparation, Thanksgiving, and Supplication.
We come to Mass to give adoration to God. Attending Sunday Mass is a concrete way to express the fact that God is most important in our lives. During the one hour at Mass we adore God interiorly in our hearts, exteriorly in our conduct, and publicly in community. God deserves our worship, and refusing to attend Sunday Mass is to disregard the right of God.
Holy Mass is a sacrifice which atones for our sins. The holy Eucharist also gives us strength to overcome sin. Some people call missing Mass on Sunday a “parent sin,” which means this sin leads to many other sins, for by missing Mass we isolate ourselves from God and from His grace, and thus do not have sufficient strength to resist temptations.
The one hour at Mass acknowledges that every good thing comes from God. All 168 hours of the week are gifts of God, and it is only right and just that we spend an hour in more intense worship of God.
As baptized Christians we participate in the priesthood of Christ. We ought to pray for people in the world. A fundamental way to exercise our baptismal priesthood is to offer the Holy Eucharist to the Father with the priest.
第三誡的「當守瞻禮日」令我們參與星期日的彌撒聖祭和放下不需要的勞動工作。
星期日被定為聖日(主日)，因為主耶穌在復活主日從死者中復活，而天主聖神也在五旬主日降臨於宗徒間。
安息日
天主教教理說：「星期日和其他的瞻禮日，舉凡一切工作或活動會影響朝拜天主，瞻禮日應有的喜樂，慈悲的工作和適當的肉體和精神上的休息，都要避免。」
從星期一到星期六我們主要忙於照顧我們肉身的需求，所以在星期日我們應該特別朝拜天主和照顧我們的靈魂。
若望哈頓神父寫道:「教會訓示，星期日應為休息日，因為我們的信仰和健康是需要週期性的鬆馳。無論如何，星期日的休息意味著所有人都有權和責任享受神性和人類律法所容許的休閑時間。他們可以利用這個休閑時間去培育他們的社交，宗教和家庭生活。」
在主日，信友應避免不必要的勞動工作，但有需要的勞動工作(為恭敬天主的事、為個人必需、或為別人的必需而作的) 則是容許的。例如: 煮食、飼養家畜、必需的公共服務、為避免嚴重經濟損失的工作，在主日都是可以做的。
朝拜日
主日參與彌撒是天主教徒的一個重要的責任。若望哈頓神父列出一些可免除教友參與主日彌撒的特殊理由：臥病在床、沒有神父主持彌撒聖祭、絕對需要履行很嚴重的責任、需要陪襯在病人牀邊或幫助一個甚為需要恊助的人。
參與主日彌撒的理由，主要有四個: 崇拜，補贖，感恩和求恩。
我們參與彌撒去崇拜天主。參與彌撒是落實的方法去表達天主是我們生命中最重要的。在這一小時的彌撒中，我們內心，外表行為上朝拜天主，在團體內公開地朝拜天主。我們是應該朝拜天主的，拒絕參與彌撒是否定天主接受公禮朝拜的權利。
彌撒是補贖我們罪過的犧牲。聖體是給我們力量去克服罪過。有人說不參與主日彌撒的罪會引發其他罪，因為沒有主日彌撒，我們便失去獲得恩寵的重要渠道，因而沒有足夠的力量去扺抗誘惑和行善避惡。
參與彌撒也就是在承認一切美善都是由天主而來的。一個星期的168小時都全是天主的禮物，所以要能用一小時深切地崇拜天主也是理所當然的。
作為一個領洗後的教友，我們參與基督的司祭職。我們應該為全世界人民祈禱。最基本的方法去行使我們的司祭職務就是偕同神父一起向天父奉獻耶穌聖體。
