Jews and Seventh Day Adventists observe Saturday as the Sabbath (strictly speaking, Jewish observance is from sunset on Friday night until sunset on Saturday night). In traditional reckoning (ignore the godless lot at the ISO!) Sunday is the first day of the week* which makes Saturday the seventh day. [ *Luke 24 v.1, John 20 v.1, etc. ] Because the Resurrection of Christ is the defining event in Christianity, the early Christians decided to observe Sunday, rather than Saturday as their sabbath, or holy day.