Why worship on Sunday?
"Sunday, the day of Christ’s resurrection and of the appearances to the disciples by the crucified and risen Christ, is the primary day on which Christians gather to worship. Within this assembly, the Word is read and preached and the sacraments are celebrated." The Use of the Means of Grace: A Statement on the Practice of Word and Sacrament, principle 6
Sunday is the original Christian feast day focused on the resurrection of Christ. It is "the Lord's Day." The name for this day of the week reflects this focus in many languages. In Spanish and Italian, for example, the name is based on the word for "lord." Unfortunately, in English and the northern European languages, the old mythological names are used (for examples, Sun, Thor, Saturn).
Scripture carefully identifies the first day of the week (Sunday, not Saturday–the Sabbath) as the day of resurrection. The women find the tomb empty on the first day of the week (Matthew. 28:1; Mark 16:2; John 20:1); the risen Christ appears first on this day (Mark. 16:9; John 20:19).
The earliest Christians gathered before dawn on the first day of the week (even while it was still a working day in the ancient world) to read from Hebrew scripture and the witness of the apostles and evangelists, and to share in the Lord's supper (Acts 20:7). From its first observance by Christians, Sunday has been a day of worship, not necessarily a day off from work. The Roman emperor Constantine dignified it as a holiday from work because it was already the special day to honor Christ in worship each week.
It is no accident that Christians assemble on Sunday for worship and especially for Holy Communion. From the first Easter, Christians have experienced the appearances of the resurrected Christ on the first day of the week. Worship on this day is a witness to Christ, his resurrection, and the hope of his return. John the Divine describes himself as being "in the Spirit on the Lord’s Day" as the context for his revelation.
Historically, Sunday is not simply a Christian version of the Jewish Sabbath. It is likely that Jewish Christians in the early centuries observed both the Sabbath (Saturday) with the Hebrews and the first day of the week–-the Lord's Day–-with Christians because each day had its own religious significance. The Sabbath finds its origin in the decalogue. The Sabbath recalls creation: " . . . on the seventh day God rested." Sunday points to the resurrection of Jesus. The Apology of the Augsburg Confession clearly argues that the Christian celebration of Sunday is not a continuation of the Sabbath observance (Augsburg Confession Article 28). Luther also taught that the appropriate behavior for Christians is to focus on the preaching, hearing and learning of God’s Word, not on rest from work. (Small Catechism: Third Commandment)
Of course, Sunday is not the only day worship is appropriate. No day should pass without praising God in Christ Jesus. Daily prayer—morning and evening—by the congregation or in the home; worship on Saturday on days of special devotion, during the seasons of preparation of Advent and Lent; and prayer before meals and upon retiring and rising are all important. But these additional opportunities for worship do not detract from Sunday's special place in the proclamation of the Christian gospel.
Worship on Sunday is an ancient and biblical tradition; it is a witness to the historical reality of Christ's resurrection. Sunday worship maintains Christian identity in either a hostile culture—as in the first few centuries of the Christian era — or in a culture that is officially supportive of the church, as in the centuries that followed Constantine. In our own day, the assembling of baptized believers to celebrate the Lord’s supper is an important evangelical witness to the world.
All things in the Christian life are carried out in faith, hope, and charity, looking forward to the glory that is yet to be revealed. This is preeminently true of the Sunday gathering of the faithful. On the Eighth Day, the perpetual First Day of a new age, this view of eternity comes into focus. Then, in a particular sense, our heavenly citizenship is clearly and unequivocally affirmed. Here we renew our allegiance each week to the Jerusalem that is above, here we are given some vision of the hope of our calling. On Sunday this is given to us not merely in homiletic exhortations to belief or catechetical declarations of the faith but in the actual living experience of a full and comprehensive worship. — H. Boone Porter, The Day of Light
The Lord’s Day proclaims the beginning of a new day (era) where Christ is Lord of time, of life and death and new life, and of a new community.