The word sabbath means “rest” and the practice of resting on the Sabbath is modelled on creation where God rested on the seventh day.
For in six days the LORD made heaven and earth, the sea, and all that is in them, and rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy. (Exodus 20:11)
So the Sabbath is the seventh day of the week, or Saturday.
However, because Jews have traditionally used sunset as the start of a new day, it is usual to observe the Sabbath from Friday sunset to Saturday sunset.
The practice of calling Sunday the “Christian Sabbath” has no biblical basis, although the practice of Christian worship on a Sunday goes back to Bible times. It seems that the mostly Jewish churches in the first century continued to meet at the synagogue on the Sabbath (Saturday), and held their Christian services on the Sunday (or possibly after sunset on Saturday).