Let's clear up some confusion. According to the Judaism of the Jews, the shabath day, the seventh day of the week, begins Friday sunset and ends Saturday sunset. The Jews begin each week day at sunset because the Pharisees of old discarded the traditional 364-day calendar and adopted the pagan lunar calendar of the heathen Babylonians (used in the Greek Seleucid Empire) which begins each day at sunset because sunset is when the moon typically becomes visible in the sky. If your calendar is lunar then your days begin at sunset when the moon comes out.
So the "Jewish" shabath day is not just Saturday. It coincides with what is now called Friday sunset to Saturday sunset. The "Jewish" first day of the week coincides with Saturday sunset to Sunday sunset.
Now the Roman way of reckoning days is to begin them at midnight, not at sunset.
On March 7, 321 AD, Emperor Constantine decreed: "On the venerable day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed" (Codex Justinianus lib. 3, tit. 12, 3; trans. in Philip Schaff, History of the Christian Church, Vol. 3, p. 380, note 1).
It must be noted that in making this change Constantine DID NOT change the shabath day; he merely made Sunday, which begins at midnight (!) in the Roman reckoning, the official day of rest for the Roman Empire. He did this in order to anathematize the "Jewish" observance of the shabath day, and this development was welcomed by Christians eager to distance themselves from the Jews. These are Constantine's own words: "Let us then have nothing in common with the detestable Jewish crowd; for we have received from our Saviour a different way" (Eusebius, Life of Constantine Vol. III Ch. XVIII).
As abayonaym (aka ebionites), we do not care what pagan names Romans gave the days of the week. Those names only mean something if you foolishly believe in pagan Roman mythology. All we care about is the fact that, prior to Constantine, what the Romans called Saturni dies ("Saturn's Day") was the first day of the planetary week in the Roman system and part of the seventh-day in the "Jewish" system. This fact cannot be overturned by any amount of special pleading. It is a fact that Pharisee Judaism's shabath day always included part of what is called Saturday BEFORE Constantine was born.
The formulation of the Roman planetary week goes back to Ptolemaic Egypt but its first historical description is by the Roman historian Cassius Dio (Roman Consul in 229 AD). According to this historian, the first day of the Roman week began with Saturday (Bonnie Blackburn & Leofranc Holford-Strevens, The Oxford Companion to the Year, An exploration of calendar customs and time-reckoning [Oxford University Press, 1999, 2003]: 566-568).
Since it was Constantine's aim to MOVE the weekly day of rest from falling on the same day that the Jews considered to be the weekly day of rest, it is clear that the true weekly shabath does include so-called Saturday. That the now popular "Jewish" method erroneously commences shabath on Friday sunset instead of correctly on Saturday sunrise does not change this fact. It also doesn't matter what pagan name pagans gave the shabath day. Every day of the week has a pagan name associated with some pagan god. This means nothing to us. We are only concerned with the POSITION of each day of the week, not the ridiculous names pagans gave them. According to Thorah and the prophets, our days of the week and our months of the year do not have names. They never did. They are numbered not named.