When and why did the early Christians change their day of worship from Saturday to Sunday?
Your question of course refers to the third commandment: “Remember to keep holy the Sabbath” or “Remember to keep holy the Lord’s Day”.
The Sabbath for the Jews was the seventh day of the week, Saturday, and a day of rest. God explained to Moses, “Six days shall work be done, but the seventh day is a Sabbath of solemn rest, holy to the Lord” (Ex 31:15).
The reason for it goes back to God’s work of creation: “And on the seventh day God finished his work which he had done, and he rested on the seventh day from all his work which he had done. So God blessed the seventh day and hallowed it, because on it God rested from all his work which he had done in creation” (Gen 2:2-3).
When did Christians change their day of rest and worship to Sunday? It came about almost immediately and spontaneously. Already in the Acts of the Apostles, written by St Luke sometime before the year 70, we read: “On the first day of the week, when we were gathered together to break bread…” (Acts 20:7). The first day of the week was Sunday and the breaking of the bread was of course the celebration of the Eucharist, or Mass. Similarly in the Didache, written probably late in the first century, we find:
“But every Lord’s day gather yourselves together, and break bread, and give thanksgiving” (14, 1a). We note here the early use of the term “Lord’s day” for Sunday.
Even though the first Christians gathered on Sundays to celebrate the Eucharist, at the beginning many of them continued to attend the synagogue on Saturdays as well.
St John Paul II writes in his Apostolic Letter Dies Domini (1998): “The apostles, and in particular St Paul, continued initially to attend the synagogue so that there they might proclaim Jesus Christ, commenting upon ‘the words of the prophets which are read every Sabbath’ (Acts 13:27). Some communities observed the Sabbath while also celebrating Sunday. Soon, however, the two days began to be distinguished ever more clearly, in reaction chiefly to the insistence of those Christians whose origins in Judaism made them inclined to maintain the obligation of the old Law” (DD 23).
The principal reason for gathering together on Sundays and calling it the Lord’s day was of course that Christ rose from the dead on that day, the first day of the week (cf. Jn 20:1). Likewise the coming of the Holy Spirit at Pentecost took place on a Sunday. But the early Christians went further and associated the first day with the first day of creation.
St John Paul II writes: “Christian thought spontaneously linked the Resurrection, which took place on “the first day of the week”, with the first day of that cosmic week (cf. Gen 1:1-2:4) which shapes the creation story in the Book of Genesis: the day of the creation of light (cf. 1:3-5). This link invited an understanding of the Resurrection as the beginning of a new creation, the first fruits of which is the glorious Christ, “the first born of all creation” (Col 1:15) and “the first born from the dead” (Col 1:18; DD 24). It is significant that as light was created on the first day, so Christ is “the light of the world” (Jn 8:12).
In the middle of the second century St Justin takes up this theme, commenting on the significance of the first day of the week being named after the sun in Latin: “We all gather on the day of the sun, for it is the first day [after the Jewish Sabbath, but also the first day] when God, separating matter from darkness, made the world; and on this same day Jesus Christ our Saviour rose from the dead” (1 Apol. 67). The early Christians referred to Christ as the “sun of justice” and so it was fitting that they would honour him on this day rather than worship the sun as the pagans did.
But Sunday is also the eighth day and as such it calls to mind the day without end which is eternal life, eternal rest, with God. St Augustine in his Confessions asks God to grant us “the peace of quietness, the peace of the Sabbath, a peace with no evening” (Conf. 13, 50).
And St John Paul II, quoting St Basil, explains that “Sunday symbolises that truly singular day which will follow the present time, the day without end which will know neither evening nor morning, the imperishable age which will never grow old; Sunday is the ceaseless foretelling of life without end which renews the hope of Christians and encourages them on their way” (cf. On the Holy Spirit, 27, 66; DD 26).
So Sunday is the weekly celebration of Easter, “the day of days”, and it should have great importance in the lives of all.