I can prove this all historically but I only have a moment this morning so I will give a highlight and if you want sources I can give them tonight.
The very early church continued to keep what you probably call the "Jewish feasts." I don't.... but that isn't important. Resurrection day itself, the Sunday after Passover, was set apart by "some" Hellenistic Jews (Greek speaking and influenced) but the vast majority continued to keep the Feasts and the Sunday after Passover is already set apart as the day one would begin their count of Sabbaths to Shavuot (Pentecost). It seems that as each year passed more and more Christians were setting aside Sunday as the day of observance and Easter Sunday as a special day to set apart... but not until after the destruction of the Temple (70AD) and the Bar Kokhba Revolt (132-134AD) did things really change. By 150AD, the "visible face" of Christianity was no longer Jewish but Greek and Sunday observance and the setting aside of the Sunday after Passover was now a majority thing. By the time you get to Constantine, he isn't making a decree to change the Sabbath, he was making law what was the practice of all non-Jewish Christians who by now are a very small minority. What we now call Easter Sunday was called Pascha (Passover) by the Christians. This can be proven historically very easily, it is even proven in Scripture. You can go to Acts 12:4 and see the word "Easter" where the Greek says Pascha. Tyndale was the first to translate pascha in that verse as Easter, whereas in other places he translated it as a word he created, "Passover." The reason for the change in Acts 12:4 is he was trying to differentiate between the pesach (sacrificial lamb and first day of Unleavened Bread) and the day of resurrection. By the way, even Constantine's "Letter on the keeping of Easter" is only translated as 'Easter' to show he was speaking about resurrection day, yet, he used the Greek word for Passover in his letter, Pascha.
