The commandments or “precepts” of the Church are laws of the Church which pertain to moral and liturgical practice and are understood as “the very necessary minimum” to be expected if one is to grow in the loving God and neighbor. Each of these precepts gives a basic starting point of what ought to be fulfilled in response to the love that God has for us and in His desire for us to share eternal life in communion with Him in heaven.
The first precept (as given by the Catechism of the Catholic Church, paragraph 2042) is “You shall attend Mass on Sundays and on holy days of obligation and rest from servile labor.” The same paragraph further explains that this precept calls us to keep every Sunday (as a commemoration of the Resurrection of Jesus) and the principal feasts of the mysteries of the Lord, of Mary, and the Saints as a way of sanctifying these days by participating in Holy Mass. It likewise includes our abstaining from work in order that we might be able to honor the Lord through our partaking rightly in the Mass.
But why the Mass? (After all, many claim Mass to be “boring,” or lacking in a personally meaningful way to relate to God, unlike through spontaneous prayers, through being in nature, etc.) In considering this question, we first point out what the Mass truly is: That Mass is the “re-presentation” of the one-time sacrifice of Jesus offered to the Father on our behalf in reparation or atonement for our sins. It is a “re-presentation” as by the sacramental offering of the Eucharist, that which was offered by Jesus once for all on Calvary (in Jesus’ crucifixion) is made present again in a non-violent and unbloody manner – that all generations until the end of time might partake of the fruits of His once for all sacrifice. That you and I do participate in Mass on many occasions is a gift wherein we are being united to the once offered and accomplished sacrifice of Jesus and from which we receive the gift of the eternal life of Jesus Christ, in His body, blood, soul, and divinity, that is made present in the Eucharist and that draws us into communion with Christ.
Practically speaking: The Mass is God’s greatest gift to us; nothing (let me repeat: nothing!) that you, I, or all of humanity together do by our own efforts is even a fraction of what God gives to us in the Mass – for no human act offers what belongs to God to offer to us. That the Mass is offered through human beings (the ordained priests, sinners though the priests are), takes nothing from God’s power to affect His gift. It is fully God’s own gift to give and we are called to “partake,” or receive from what He Himself does on our behalf – as His desire for us is that we would share His eternal life, now and forever. Such participation is, of course, demanding of the utmost reverence as is fittingly rendered onto God.
When it comes to the keeping of this precept, therefore, know that our participation in Mass on Sundays and holy days is as a gift of God and it is the truest way to remain in relationship with Him. Formally, we can think of it as the renewing and ratifying of the new, eternal covenant in the Blood of Christ. More simply, we can also think of it as right worship offered to God who gave us life and died for us when we were his enemies (see Romans 5:6-11) so that we could be once more in union with Him – a union that is renewed and strengthened with every right participation in the Eucharist. Spontaneous prayers, a day on the lake, a hike on the trail, or a visit to any other place may be beautiful; but none of these are God’s gift of communion to us without which we have no life within us (see John 6:53).
For further reading: In the Catechism of the Catholic Church, paragraphs 2041 - 2043 gives the brief definition and the official formulations of the five precepts.