Arthur Weigall says "Many long standing pagan festivals were incorporated and given significance - for example, Christianity had abolished the Hebrew Sabbath of Saturday so made Sunday their day of worship, partly because it was the day of the resurrection, but largely because it was the weekly festival of the sun for its main competing rival, Mithraism. Also, December 25th was the birthday of the sun-god, Mithra, so this date was brought in and called "Christmas," with full knowledge that Jesus was not born on this day." (The Paganism in Our Christianity)