Well that question has alot of variables to it. Sabbath Day for a Jewish Person is different than the Sabbath of a Catholic or Protestant. As for a Christian, Jesus stated to live everyday as if it were the Sabbath. But initially it was a day of rest to cast aside all things of the world and to focus on God. Historically it was on Saturday and then it was changed to Sunday. It is a reminder of the creation story "and on the seventh day god rested." It's more just to honor god and to take time out of the week to focus on him. However there could be a sabbath day in other religions as well.