You’ve done a great job explaining. This concept is still prevalent even today in Judaism. The Jewish Sabbath begins at sundown on Friday, extends through Friday night and all day on Saturday, and then ends Saturday evening at sunset. Sunset to sunset.
The concept is derived from the Book of Genesis, Chapter 1:5 - “…And there was evening, and there was morning, one day.”
The Christian Sabbath, however, is celebrated on Sunday. I believe this was instituted by Constantine, but someone can correct me if I am mistaken. The first Christians were Jews, so they celebrated the Sabbath from Friday at sunset to Saturday at sunset, then later gathered on Sunday for what were the earliest vestiges of the Mass in remembrance of Christ.
As the Gentiles came into the Church, the Gentiles were not required to follow the prescriptions of the Torah, such as keeping kosher, observing the Jewish Sabbath and festivals, wearing tefillin, etc.
Christians were released from many of the rules and guidelines outlined in the Torah. You see this debate in the New Testament where the early founders of the Church debated the topic as to what aspects of the Torah the Gentiles would be require to follow. The thought at that time, as I have read from some theologians, was that if a newly converted Christian later wanted to become a Jew, he could pursue that more rigorous spiritual path at a later date. In the interim, the Gentile Christian was not required to follow the strictures of the Torah.
You sometimes see these Gentile laws referred to as the “Noahide laws” which were the laws that the Gentiles were proscribed to follow, which basically were a handful of very basic laws similar to those found in the Ten Commandments. The Torah, by contrast, contains some 600+ laws that the Jewish people are required to follow.