Actually, Saturday is the seventh day. Christians moved the holy day to Sunday because it's the day Jesus rose from the dead. While Christians mainly treat the day with the same respect as the Jewish people do the Sabbath, most also recognize Jesus was chewed out for healing people on the Sabbath day, and replied to the detractors; "If your ox is stuck in a mire on the Sabbath, would you not still pull it out? Therefore, it is lawful to do good on the Sabbath; the Sabbath was made for man, not man for the Sabbath. Therefore, the Son of Man is Lord even of the Sabbath." So, that's why apparently BYU is unique among the NCAA in absolutely refusing to play on that day.