Acts 20:7

And upon the first day of the week, when we were gathered together to break bread, Paul discoursed with them, intending to depart on the morrow; and prolonged his speech until midnight.

First day of the week when the disciples came together to break bread ... This emphatically states the purpose of Christian assemblies on Sundays throughout history, that purpose being for the observance of the Lord's supper. As Lange said, "Luke's language here plainly indicates that this day (Sunday) was precisely one on which assemblies for religious services were customarily held."[7] Harrison complained that "We are not told when or how the practice of Sunday worship arose in the church";[8] but one does not need to seek any later than the day of the resurrection of our Lord for the beginning of it. On successive Sundays, Jesus appeared to the apostles on the day he arose from the grave (John 20:19), Thomas being absent; and again on the following Sunday (Thomas present) (John 20:26) he appeared to them again. There can be little doubt that Sunday services of Christians began with those two appearances of our Lord in their assemblies on successive Sundays.

Pliny's letter to the Emperor Trajan, written in the shadow of the apostolic age (112 A.D.), declared of the Christians that:

It was their habit on a fixed day to assemble before daylight and sing ... After this was done, their custom was to depart and meet again to take food, but ordinary and harmless food.[9]

It is easy in Pliny's report to observe a reference to the Lord's supper; and the significance of "on a fixed day" is therefore of the very greatest magnitude. The Christians, from earliest times, had the habit of meeting for the Lord's supper on "a fixed day," and Acts 20:7 identifies that day as "the first day of the week," Sunday.

To break bread ... as Dummelow noted, means "to celebrate the Lord's supper."[10] In fact, it would be impossible to understand this as a reference to anything else.

Paul discoursed unto them ... Even the address of so distinguished an apostle as Paul took second billing on that occasion, the primary purpose having been to observe the Lord's supper; that is why no ordinary meal can be understood of this "breaking bread."

Continued his speech until midnight ... Perhaps it should be noted as Lange said, that "The example of Paul affords no excuse for sermons that are of immoderate length!"[11]

[7] John Peter Lange, Commentary on Acts (Grand Rapids, Michigan: Zondervan Publishing House), p. 368.

[8] Everett F. Harrison, Wycliffe Bible Commentary (Chicago: Moody Press, 1971), p. 459.

[9] Henry Melvill Gwatkin, Selections from Early Writers (Westwood, New Jersey: Fleming H. Revell Company), p. 29.

[10] J. R. Dummelow, op. cit., p. 846.

[11] John Peter Lange, op. cit., p. 370.