A Week of Catholicity I: The Sunday Obligation
"Catholic": of or including all Christians • of or relating to the historic doctrine and practice of the Western Church.
Christians go to Church on Sunday.
First, because Sunday is the day on which the Church has always honored the historical Resurrection of Jesus on the first day of the week. (More on this blog's explanation as to why we believe in the Resurrection here) In this regard, the Church's Sunday observance is vastly different from the Jewish Sabbath. The Sabbath constituted one of God's gracious provisions whereby His people might mark their reliance on His covenantal promises; but on Sunday, Christians commemorate the Resurrection, whereby God Himself has entered into His own rest, and has marked the consummation of His creation, salvific work, and His covenant in raising Jesus from the dead. In the words of St. Ignatius, "Christians no longer observe the Sabbath, but live in the observance of the Lord's Day, on which also Our Life rose again."
Secondly, and inconveniently in an anti-establishment culture, Christians go to church on Sunday because we recognize our gathering to commemorate the Lord's Resurrection as an enactment of the ontological reality of the single Body for whom Christ died, and for whom He intercedes. The "gathering" of His people seems to be something constant in the will of God, from Sinai to Gathsemanee and Pentecost, for their benefit, growth, and protection, and for the greater glory of His name and fame; and the "gathering" of the Church anticipates our final gathering together when we are called to our marriage to Christ at the last day. If we believe it for later, we should practice for it now. In gathering together on the day that commemorates the Resurrection, Christians adhere to the practices of the earliest church, as indicated in Acts 20:7, I Corinthians 16: 2, and in Revelation 1:10. The earliest manual of church liturgy and practuce, the Didache, enjoins Christians of around A.D. 60 to "on the Lord's Day come together and break bread. And give thanks/offer Eucharist, after confessing your sins, that your sacrifice may be pure." Almost as early, we find in the Epistle of Barnabas (ca A.D. 80-120) the following: "wherefore, also, we are sure to keep the eight day (i. e. the first of the week) with joyfulness, the day also on which Jesus rose again from the dead."
More of the official line on the obedience due our Lord in observance of Sunday here.
Finally, in my own humble words, we go to Church on Sunday because Jesus is there. I have heard far too often that one can encounter Christ "just as easily" on the golf course or in one's home, if one Chooses to Sleep In. That may very well be; the Holy Spirit is all over the place. But the sad thing is that Christ has PROMISED to meet His people when they are gathered in His name. If we know for a fact that He will show up at our Church on Sunday morning- Matthew 18:20, "for where two or three are gathered together in my name, there am I in the midst of them," why would we be so rude as to avoid His guaranteed presence? It's JESUS, people. Those of us in the liturgical traditions, furthermore, know that He is REALLY there, in as much as He has assured us that His very body and blood will be provided in accessible forms for us at a certain time and a certain place. The God almighty, the compelling and immenently attractive Jesus, waits for us on the altar, and we prefer to sleep in? Those of us in the more Updated Versions have also got little excuse, because in our hymns and preaching (though pale substitutes for His communion, if you ask me) He has also promised to inhabit our praises (Psalm 22:3).
In sum then: we go to Church on Sundays because that is where Jesus has told us that we can find Him on Sunday mornings.
BTW, the churchladies at "Be Not Conformed" recently had some nice suggestions on how to order one's life around Sunday observance here.