We can say, first, that Jesus may very well have instructed the apostles at greater length about what he wanted them to do in celebrating the Eucharist.
He would probably not have done it in the Last Supper itself because there he was celebrating the great feast of Passover, in which all the rites were prescribed. He would more likely have done it sometime after his resurrection.
In general, there is not much written in detail about the early Mass, among other reasons because the early Christians lived what was called “the discipline of the secret.”
That is, they were encouraged not to share with non-believers their intimate teachings and practices. For example, even the catechumens, who were taking instructions to become Christians but were not yet baptised, were allowed to remain at Mass only for the Liturgy of the Word and they had to leave for the anaphora or Eucharistic Prayer.
Among the reasons given by the Fathers of the Church for the “discipline of the secret” were that the mysteries of faith were so great that reducing them to words would be a profanation, that the church should not risk exposing the mysteries to ridicule by those who would not understand them, and that the sacraments were intimate matters and, like intimacy in marriage, they were not suitable subjects for public conversation. In any case, we do know something about how the Eucharist was celebrated at that time.
As regards the name, the first Christians called the Mass the “breaking of the bread.” We read, for example, in the Acts of the Apostles: “They devoted themselves to the apostles’ teaching and fellowship, to the breaking of bread and the prayers” (Acts 2:42).
Moreover, they celebrated the Eucharist especially on Sunday, “the Lord’s day” (cf. Rev 1:10), the day of Christ’s resurrection and the first day of the week: “On the first day of the week, when we were gathered together to break bread, Paul talked with them…” (Acts 20:7).
Even though the first Christians had been Jews and were used to attending the synagogue service on Saturday, the Sabbath, early on they began to celebrate the Eucharist on Sunday because it was the day of Christ’s Resurrection, the first day of creation and also the day of Pentecost.
As regards the place where they celebrated the Eucharist, it was in their homes, since they had not yet built churches.
In the passage we just read, Paul and the others were in a house, on the third story, where there were many lights (cf. Acts 20: 8-9).
While the early Christians began immediately to celebrate the breaking of the bread, they did not make a complete break from their attendance at Jewish ceremonies.
For example, believers in Jerusalem continued to go to the Temple daily (cf. Acts 2:46, 3:1) and the apostles often attended and preached in the synagogue in order to proclaim Jesus Christ to the Jews as their Messiah (cf. Acts 13:13ff; 14:1; 16:1).
As regards the text of the Eucharist, the “Holy, holy, holy”, which comes from Isaiah 6:3 and was the starting point of the Kedusha used in the synagogue service every Sabbath, appears early in Christian literature and it is mentioned in the Book of Revelation in the description of the heavenly liturgy (cf. Rev 4:8).
Another carryover from the Jewish liturgy is the Alleluia, from the Hallel Psalms which concluded the Passover meal, and the refrain “Blessed is he who comes in the name of the Lord!”
It seems clear too that the first part of the Mass, the Liturgy of the Word, followed the order of the synagogue service, with its readings, psalms, prayers, explanation of the readings, etc.
Also, the order of the earliest eucharistic prayers followed the sequence of prayers in the Jewish Passover meal.
