When I was a child, I often argued with my grand mother when she told me that by the vespers bell a new day starts. For her, traditional the next day begins with the vespers and not at 12:00 p.m. midnight. I couldn’t understand that, but later I read that in older times it was so conceded.
And having that in mind, I tried to figure out an explanation to my question, and you are invited to enjoy reading to which conclusion I came.
The first Christians were Jews, or of Hebrew descendance, and they celebrated Saturday like all the rest of their kinship. Later, when the Christians grew in number, and in an attempt to keep the Jews under control, the Rabbis excluded them from the regular Jewish synagogue mass. At least they were not allowed to make the mass together with the rest of the Jews, but postponed it at later hours, until it became quite regular that they will met at the evening of Saturday.
We have also the influence of the follows of Mithra, which were quite popular among the military classes, who celebrated Sunday. Mithraism was accepted by the Romans, but not Christianity; we all know how they were sometimes persecuted or punished with death. To avoid this, and since both religions, Mithraism and the infant Christianity, celebrated Sunday, Christians started to go to Church on next day's morning (Sunday). After all, it did happen in the same day; as I said before, the vespers and next morning belongs traditionally to the same day. So, by the third century was quite common that a lot of people worshipped their different gods on Sunday and only the Jews persisted on Saturday.
How does my explanation sound?
