First day worship is not Biblical.
Matt 26:17
the first day of the feast of unleavened bread the disciples came to Jesus, saying unto him, Where wilt thou that we prepare for thee to eat the Passover?
KJV
There is a translation problem here as Passover is not part of Unleavened Bread as the first day of Unleavened Bread is a high holy day and Passover is not. It is a day to remember or of memorial but not a holy day. This is the first of 12 references to first day in the New Testament. None of which have any reference to first day worship.
Matt 28:1
28:1 In the end of the sabbath, as it began to dawn toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre.
KJV
This is the second.
Mark 14:12
12 And the first day of unleavened bread, when they killed the passover, his disciples said unto him, Where wilt thou that we go and prepare that thou mayest eat the passover?
KJV
Christ is keeping the laws of His Father.
Mark 16:2
2 And very early in the morning the first day of the week, they came unto the sepulchre at the rising of the sun.
KJV
Nothing about worship here either.
Mark 16:9
9 Now when Jesus was risen early the first day of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils.
KJV
Nothing about worship here either.
Luke 24:1
24:1 Now upon the first day of the week, very early in the morning, they came unto the sepulchre, bringing the spices which they had prepared, and certain others with them.
KJV
John 20:1
20:1 The first day of the week cometh Mary Magdalene early, when it was yet dark, unto the sepulchre, and seeth the stone taken away from the sepulchre.
KJV
John 20:19
19 Then the same day at evening, being the first day of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
KJV
At sunset on the seventh day Sabbath starts the first day which this was and they were there were there for fear of the Jews not to worship.
Acts 20:7
7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
KJV
We have the same problem here as teaching until midnight is just after the Sabbath and it was most likely a meeting of the elders of the new Church, who the Jews would have been targeting, as they would not go after the rank and file only the elders. He was teaching them the laws of God.
Acts 20:18
18 And when they were come to him, he said unto them, Ye know, from the first day that I came into Asia, after what manner I have been with you at all seasons,
KJV
1 Cor 16:2
2 Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come.
KJV
Paul is not even there and many collections were done on the first day of the week.
Phil 1:5
5 For your fellowship in the gospel from the first day until now;
KJV
This is all of the references to the first day and I see no worship services in any of them. Plus there is no foretelling of that change in the Old Testament, which is the telling tale. All other changes were fore told many times and centuries before there implementation.
humble servant of the Lord God Most High