First Precept of the Church.
"The First Precept of the Church is simply an interpretation, or application, of the Third Commandment of God in accordance with Apostolic tradition, inasmuch as it prescribes assistance with due attention and devotion at the Holy Sacrifice of the Mass, as the work best calculated to sanctify the Sunday - the minimum required by the Church for that purpose - and inasmuch as in a certain sense it puts other festivals on the same footing as Sundays.
As much has already been said on the subject of assistance at Mass on Sundays and festivals, it will be enough here to recall to mind that the reason why Mass has been selected as the principal duty by which to sanctify Sundays and festivals was because these days are consecrated to God's worship, and because the Sacrifice of the Mass is the holiest and most salutary of all acts of divine worship.
It should also be observed that the Precept obliges under pain of mortal sin all those who have attained the use of reason and who are not hindered from being present by good and sufficient reasons. And not alone must we be present at Mass, but we must be present at the whole Mass - that is, from the beginning to the end, - and along with this, imbued all the time with the spirit of attention and devotion. Hence this Precept is sinned against by absence from Mass either wholly or partially on Sundays or holydays, or, if present, by disrespect.
Moreover, while any Mass suffices, it is the Church's desire that the faithful should assist at the parochial Mass [i.e. at your local parish] at which a sermon is preached; and the reason is because she regards the Word of God as a powerful means of salvation - a means instituted by God Himself; because she considers it highly important to recall to mind the sacred truths of faith and to listen to exhortations to a good and Christian life; and because she regards it as tending to mutual edification: "He that is of God, hears the words of God." [John 8:47DR; NAB]
But not only does the First Precept of the Church direct us to sanctify Sundays - it also commands us to keep in precisely the same way as we should keep Sundays, the holydays or feasts instituted by the Church in honor of our Lord, the Blessed Virgin, and the Saints.
Regarding such days, [i]t will be quite enough... to point out that their general object is God's honor and glory on the one hand and the instruction and edification of the faithful on the other.
Thus, as the mysteries of the life of Jesus Christ are the source of all graces and the foundation of our salvation, the remembrance of them must naturally excite in us an increase of faith, of fervor, of love, of gratitude, of confidence; and hence it is that the Church puts them before our mind year after year, brings them home to us and presents them to us as if the particular mystery being celebrated were actually taking place that very day, so vividly does she transport us in spirit to the time and place in which they were accomplished And in thus bringing the objects before our minds in this vivid way, her purpose is that they may deeply impress us, and reanimate our faith and piety as keenly as possible."
Right Rev. John Hagan, A Compendium of Catechetical Instruction, Vol. III, The Comandments. Brown and Nolan Ltd., Dublin. Imprimatur, 1928. pp. 523-24. This text, and the quote below, are presented for religious and educational purposses only. No other use is intended or permitted. 17USC107.
Catholic Belief - Examination of Conscience