The phrase, ‘first day of the week’, as found in the New Covenant writings, is the Greek mia Sabbaton (except for Mark 16:9. This is another issue altogether, which will be dealt with later). This phrase literally means, one Sabbath or first Sabbath. It doesn't make much sense by itself, until we realize it is what is called, a Hebraism. This simply means that it is a Greek term used to convey a Hebrew concept or idea. Ok, so what is the Hebrew concept? It is the Hebrew phrase, echad Shabbat. This means, first of Sabbath, or more correctly, the first in a Sabbath's cycle. The fact that the word sabbaton can be translated as ‘week’ is clearly seen in Luke 18:12, “I fast twice in the week, I give tithes of all that I possess.” The word here translated as ‘week’, is the Greek word sabbaton, the word for Sabbath.
In the Jewish weekly calendar, the Sabbath was the only day of the week that had an actual name. Every other day was counted in relation to the Sabbath. The first day of the Jewish week was known as the first in a Sabbath's cycle, the second day, second day in a Sabbath's cycle, and so on. Also, understanding that the Jewish day began at sunset, the echad Shabbat (or mia sabbaton in Greek) would begin at sunset, as the weekly Sabbath was coming to a close. The Jewish day would technically begin with the appearance of the first three stars in the sky.
In our present calendar, Sabbath begins at sunset on Friday, and closes at sunset on Saturday. The Jewish first day of the week would begin at sunset on Saturday. However, in the first century, the Jews would not and did not use the Western/Roman days. These names were pagan and the Jews would have none of that. In other words, the Jews did not adjust their weekly calendar to fit the Roman calendar, but just the opposite.
It should also be noted here, that the day before and the day after the Sabbath, were so connected to it that they were thought of as belonging to the Sabbath. For instance, the day before the Sabbath was known as Preparation , or 'erev Shabbat'. The day after the Sabbath was known as the termination of Shabbat, or 'Motzae Shabbat'. The Sabbath is so revered that they treated it as an honored and distinguished guest, actually referring to it as QUEEN SHABBAT. They would make elaborated preparations for it, and when it was time for it to depart, they desired to extend it as far as possible.
The day before the Sabbath was connected to it for its preparation, and the day following the Sabbath was connected to it for its termination. The sixth day belonged to the Sabbath, as well as did the first day. The evening following the Sabbath's termination, known as Motzae Shabbat, was observed with a meal, singing and prayer, and called Havdalah. This can be seen in Acts 20:7. “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.”
The New English Bible is one of the few that views this passage in its proper context. It says,
On the Saturday night, in our assembly for the breaking of bread, Paul, who was to leave the next day, addressed them, and went on speaking until midnight.
The Jewish New Testament translate Acts 20:7,
On Motza’ei-Shabbat, when we were gathered to break bread, Sha’ul addressed them. Since he was going to leave the next day, he kept talking until midnight.
This assembly referred to here in Acts 20:7, is clearly the Havdalah service. An especially strong indication of this is the fact that it states that Paul continued to teach until midnight. If this were a Sunday morning meeting as most claim, Paul’s preaching would have started in the morning and lasted until midnight and beyond. I know that Paul is considered a he-man, but that would be a feat of mega-proportions, worthy of recognition by Ripley’s Believe or Not.
If we keep this verse in its original Jewish context, what we have here is the Messianic Community at their usual meeting, terminating the Sabbath, with the Havdalah service. This service would have commenced as the Sabbath was ending and the first day of the Jewish week was commencing. Stallings, in his book Rediscovering Passover: A Complete Guide for Christians, says, “The community that gathered on Friday evening as the people of God, gathered again on Saturday evening as the body of Christ” (p. 284).
This reckoning of the beginning of the first day of the week at sundown on Saturday seems to have been recognized and continued for several centuries. The Catholic Encyclopedia states, “As with the Jewish Sabbath, the observance of the Christian Sunday began with sundown on Saturday and lasted till the same time on Sunday. The method of reckoning the Sunday from sunset to sunset continued in some places down to the seventeenth century, but in general since the Middle Ages the reckoning from midnight to midnight has been followed.”