"The First day of the Week" in the bible has been misunderstood and the day Christians are to worship and about the sabbath. When I looked at the part of the sentence First day of the week in Greek I was taken aback by what it meant in. But we have to go back to the history of the Israelites and what they were commanded to do, to get an understanding of this. It will make more sense further down.
Now First day of the Week is mentioned eight times in the Brit Hadasha. On these scriptures the church has based their reason for gathering on Sunday.
Mathew 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.
Mar 16:2 And very early on the first day of the week, they came to the tomb when the sun had risen.
Mar 16:9 And having risen early on the first day of the week, He appeared first to Miryam from Maḡdala, from whom He had cast out seven demons.
Luk 24:1 And on the first day of the week, at early dawn, they came to the tomb, bringing the spices which they had prepared,
Joh 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.
John 20:19 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.
Act 20:7 And on the first day of the week,the taught ones having gathered together to break bread, Sha’ul, intending to depart the next day, was reasoning with them and was extending the word till midnight.
When a word is italicized it was not originally there and was placed to give a better understanding, but honestly should be left out. So the verse should say first of the week. So let's look at the word Week in the Greek for each verse. Mathew 28:1; Mark 16:2; Mark 16:9; Luke 24:1; John 20:1; John 20:19; Acts 20:7; 1 Cor. 16:2:
G4521 σάββατον sabbaton sab'-bat-on Of Hebrew origin [H7676]; the Sabbath (that is, Shabbath), or day of weekly repose from secular avocations (also the observance or institution itself); by extension a se'nnight, that is, the interval between two Sabbaths; likewise the plural in all the above applications: - sabbath (day), week.
Sabbaton, of Hebrew origin from the Sabbath. It is the interval between two Sabbaths. Sabbath, what two Sabbaths? Hold on. The verse is really saying the First of the Sabbaths. What Sabbaths? Okay, here is where we need the history. In the Old Testament, there are Feasts that Yahweh gave to His people. They are called His Feasts. After Passover, the first sabbath after Passover is Firstfruits and then the counting of the Sabbaths to Pentecost. Pentecost is fifty days from the Sabbath after Firstfruits.
They had to count seven sabbaths to Pentecost.
Lev. 23:15,16 And ye shall count unto you from the morrow after the sabbath, from the day that ye brought the sheaf of the wave offering; seven sabbaths shall be complete: Even unto the morrow after the seventh sabbath shall ye number fifty days; and ye shall offer a new meat offering unto the LORD.
These Sabbaths became known as First Sabbath, Second Sabbath just like in:
Luke 6:1 As it came to pass on the second sabbath after the first, that he went through the corn fields;
On the second sabbath after the first in the Greek is defined as:
G1207 δευτερόπρωτος deuteroprōto sdyoo-ter-op'-ro-tos From G1208 and G4413; second first, that is, (specifically) a designation of the Sabbath immediately after the Paschal week (being the second after Passover day, and the first of the seven Sabbaths intervening before Pentecost): - second . . . after the first.
So, what does this prove you ask? Well, the belief that everyone holds that Sunday is the day that they were gathering because that is when the New Testament Christians are now supposed to gather. It proves that they were gathering not because of a new thing but because it was what they were taught out of keeping His Feasts, from Passover to Pentecost. This is why we can't just read the bible. We have to study it and look up every word whether Greek or Hebrew. We don't want to be the blind being led by the blind. I don't want to fall and then live in a ditch. Let's prove ourselves worthy. The way it is translated into English just proves the saying "lost in translation" and proves Sunday has nothing to do with anything.