It is a common misconception that Catholics or Constantine changed the day of worship to Sunday. Sunday worship was observed in Acts 20:7.
This is patently false. The day here is the first Sabbath of the seven Sabbaths of the counting to the Feast of Pentecost...
The Diaglott has it...
Act 20:7 In and the first of the sabbaths, having been assembled of us to break bread, the Paul discoursed to them, being about to depart on the morrow; continued and the discourse till midnight.
The word translated here week is Sabbaton.
Here is what Dr. Bullinger has to say about it...
Acts 20:7
first, &c. = first day of the sabbaths, i.e. the first day for reckoning the seven sabbaths to Pentecost. It depended upon the harvest (Deu_16:9), and was always from the morrow after the weekly sabbath when the wave sheaf was presented (Lev_23:15). In Joh_20:1 this was the fourth day after the Crucifixion, "the Lord's Passover. "Compare App-156. This was by Divine ordering. But in A.D. 57 it was twelve days after the week of unleavened bread, and therefore more than a fortnight later than in A.D. 29.
the disciples. The texts read "we".
came together = were gathered together, as in Act_20:8.
break bread. See note on Act_2:42.
preached. Greek. dialegomai. Often translated "reason". See note on Act_17:2.
unto = to.
ready = being about. Same as in verses: Act_20:20, Act_20:3, Act_20:13, Act_20:38,
depart. Greek. exeimi. See note on Act_13:42.
continued = was extending. Greek. parateino. Only here.
his speech = the word. Greek. logos, as in Act_20:2.
Here is the note on Acts 2:42...
Acts 2:42
And. This and the "ands" in following verses give the Figure of speech Polysyndeton. App-6.
continued stedfastly. Greek. proskartereo. See note on Act_1:14.
doctrine = teaching.
fellowship = the fellowship. Compare Act_2:44.
breaking of bread. This was the common meal. Compare verses:##Act_44:46 and Mat_14:19. Isa_58:7.
breaking. Only here and Luk_24:35.
prayers = the prayers. App-134. Compare Act_3:1.