Historically the Sabbath was on Saturday, the last day of the week. Saturday is still the Jewish Sabbath. But when the Roman Emperor Constantine converted to Christianity, he made his entire nation do the same. They had previously been pagan and celebrated the sun on the first day of the week. SO to make it easier for his people the Emperor made the first day of the week “Sunday” the Sabbath for Rome. It became a tradition, and now all of Christianity celebrates the Sabbath on Sunday, even though it is technically the first day of the week.