Hi,
The Sabbath is not really a day per se. Sabbath simply means “seven”, a day of rest. It does not mean Saturday. The Jews observe the Sabbath (their holy day of obligation) starting at dusk on Friday, and they observe Sabbath until dusk Saturday. (It is traditionally observed when the first star appears in the night sky).
We Christians observe our “sabbath” (day of rest, honoring God), on Sunday, and like the Jewish tradition (our roots are Jewish after all), we begin our sabbath on Saturday evening, ending Sunday evening. A day with the Jewish tradition always began in the evening and one day was counted not as a 24 hr period, but inclusive with that particular day. That is why when Jesus was crucified, on a Friday, the hour of the Sabbath was approaching and He had to be quickly buried (placed in the tomb). This day was counted as 1 day. Saturday, the continuation of the Sabbath, was the 2nd day. Sunday, when our Lord rose from the dead (the resurrection), was the third day, and this is what we call, and what Paul called, The Lord’s day.
As Christians, we celebrate Mass on the Lord’s day just as the Apostles did and have since then. Our Priests have their “sabbath” (day of rest) typically on Mondays, since they are “working” by celebrating Mass for the celebrants every day, except Mondays.
So, as we see, sabbath is the seventh day on which we rest. It does not mean a named day, such as Saturday.
God bless you all,