Why Do Christians Worship on Sunday?

In a recent Seventh Day Adventists seminar, I learned that Saturday is the true Sabbath and that it has not changed. Why, then, do Christians worship on Sunday and not on Saturday?

Christians are not bound to practices the ceremonial Sabbath of the Old Covenant. Let me state several reasons why from the New Testament.

Christians are not required to worship on the Sabbath day and the New Testament nowhere commands them to observe it.
The ceremonial Sabbath was specifically a sign of the Old covenant (Ex. 31:16-17; Num. 9;14; Ezek. 20:12) and Christians, now under the New Covenant, are not bound by the Old Covenant (Heb. 8). Jesus fulfilled the law, doing away with its ceremonial obligations.
If there is one day in seven that is emphasized in the life of the early church, it is Sunday, on which day Jesus rose and appeared to his disciples after his resurrection, the Holy Spirit came at Pentecost, Paul commands believers to set aside money, the early church met to worship (Acts 20:7)—the only record of any worship service in progress in the New Testament. In addition, the apostle John calls Sunday the Lord’s Day (Rev. 1:10).
The Jerusalem Council would have imposed the Sabbath observance on Gentiles believers if it was still required (Acts 15).
Of all the sins that Paul and other New Testament writers list, the breaking of the Sabbath is not among them. This is significant only in light of the fact that epitome of rebellion in the Old Testament from God’s point of view is the breaking of his Sabbath, which was the main reason for the Babylonian captivity (Ezek. 20:13-26).
It would appear, on the basis of Colossians 2:16, which specifically mentions Sabbath days, and Galatians 4:10-11, which alludes to them, that Paul rebuked his audiences for thinking that God expected them to observe ceremonial Sabbaths. Clearly Paul did not believe that the keeping of the seventh day was binding for the Christian.
At the very most, Paul taught that observing the ceremonial Sabbath was strictly a matter of Christian liberty (Romans 14:5) and no more.
From a purely historical perspective, which of course holds not authority, the early church Fathers, from Ignatius to Augustine, believed that the Sabbath had been abolished and that the first day of the week was the day when Christians should meet for worship.