Here's part of chapter eleven, "The Change of Sabbath as Evidence for Christ's Resurrection":
One monumental indication of the resurreciton of Christ concerns the Sabbath. In the first few years of Christianity, all the Christians were Jews (Acts 10:45), 11:18). The Jewish Sabbath has always been kept on Saturday, the seventh day of the week. Yet the early Christians, who were Jewish, met for worship on the first day of the week, Sunday. This was considered the Lord's Day (Rev. 1:10), because Jesus had been resurrected on the first day of the week (Mark 16:9, Luke 24:1-7, John 20:1, 19).
The practice of assembling together on the first day of the week began the week after Christ's resurrection had taken place (John 20:26). The practice of gathering together on the first day of the week is also mentioned in Acts 20:7 and I Corinthians 16:2.
To change a tradition such as the day of sabbath among Jews would ordinarily have been exceedingly difficult. The Jewish sabbath day is observed in obedience to the fourth commandment, "Remember the sabbath day, to keep it holy. Six days you shall labor, and do all your work; but the seventh day is a sabbath to the LORD your God; in it you shall not do any work, you, or your son, or your daughter, your manservant, or your maidservant, or your cattle, or the sojourner who is within your gates; for in six days the LORD made heaven and earth, the sea, and all that is in them, and rested the seventh day; therefore the LORD blessed the sabbath day and hallowed it" (Ex. 20:8-1).
The letter of Ignatius of Antioch to the Magnesians, chapter 9, refers to observation of the Lord's Day rather than the traditional Sabbath, in honor of Christ's resurrection: "How, then, shall we be able to live apart from Him, seeing that the prophets were His disciples in the Spirit and expected Him as their Master, and that many who were brought up in the old order have come to the newness of hope? They no longer observe the Jewish Sabbaths, but keep holy the Lord's day, on which, through Him and through His death, our life arose" (Ignatius of Antioch, "Epistle to the Magnesians," chapter 9, reprinted in Francis Glimm, THE APOSTOLIC FATHERS [Washington: The Catholic University of America Press, 1962], pp. 98-99). This letter to the Magnesians is one of seven letters writen by Ignatius of Antioch before he was thrown to the beasts in the Flavian amphitheatre in Rome in the second half of the reign of the Emperor Trajan (A.D. 98-117).
The Epistle of Barnabas, chapter 15, also mentions this practice: "This is why we also observe the eighth day with rejoicing, on which Jesus also rose from the dead, and having shown himself ascended to heaven." (Epistle of Barnabas, chapter 15, reprinted in Edgar J. Goodspeed, THE APOSTOLIC FATHERS [New York: Harper and Brothers, 1950], p. 41). The Epistle of Barnabas, it is agreed among scholars, could not have been written later than A.D. 135 and might have been written as early as A.D. 70 or 71.
In the APOLOGY TO CAESAR of Justin Martyr, chapter 67, we read, "But "The Lord's Day" is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead" (Justin Martyr, THE FIRST APOLOGY, chapter LXVII, reprinted in Alexander Roberts, ANTE-NICENE CHRISTIAN LIBRARY [Edinburgh: T. and T. Clark, 1867], vol. II, p. 66). Justin Martyr died in A.D. 165, when he suffered martyrdom in the reign of Marcus Aurelius, according to CHRONICON PASCHALE.
In the DIDACHE, or the TEACHING OF THE TWELVE APOSTLES, we read, "And on the Lord's Day, after you have come together, break bread and offer the Eucharist, having first confessed your offences, so that your sacrifice may be pure" (THE DIDACHE 14:1, reprinted in Francis Glimm, THE APOSTOLIC FATHERS [Washington: The Catholic University of America Press, 1962], p. 1820). The DIDACHE was held in great repute by the early church, and was probably in circulation among the churches prior to A.D. 70.
In his ECCLESIASTICAL HISTORY, Book III, chapter 27, Eusebius wrote, "They observed the sabbath and the rest of the disciples of the Jews just like them, but on Sundays they performed ceremonies like ours in commemoration of the Lord's Resurrection. Therefore, because of such practices they recieved their name, since the name of Ebionites signifies the poverty of their understanding, for the poor man is called by this name among the Hebrews" (Eusebius, ECCLESIASTICAL HISTORY, Book II, chapter 27, reprinted in Roy J. Deferrari, EUSEBIUS PAMPHILI ECCLESIASTICAL HISTORY [Washington: The Catholic University of America Press, 1953], p. 184).
Pliny the Younger, whom Trajan had sent to govern the province of Bithynia in Asia Minor, wrote in a letter in A.D. 112 to the Roman Emperor about the practice of Christians in Bithynia on this "fixed day": "They had met regularly before dawn on a fixed day to chant verses alternately among themselves in honour of Christ as if to a god, and also to bind themselves by oath, not for any criminal prupose, but to abstain from theft, robbery and adultery, to commit no breach of trust and not to deny a deposit when called upon to restore it" (Plinius Caecillius Secundus, C., Ep. X. 96, reprinted in Betty Radice, Pliny LETTERS AND PANEGYRICUS [Cambridge, Mass.: Harvard University Press, 1969], p. 289.
As can be seen from these examples, references from early sources concerning the practice of the first Christians, most of whom were Jewish, establish that they had established the first day of the week as the sabbath day in commemoration of the resurrection. It would be extremely difficult to find any other explanation for this abrupt change in the tradition of these people.