Reindeer are associated with Christmas because they are believed to have been used by Saint Nicholas to deliver gifts to children. In the 1800s, German writer and poet Jacob Grimm wrote a story called "The Children's Christmas Tree" which featured a reindeer who helped to bring presents to children. This story was adapted into "The Night Before Christmas" by Clement Clarke Moore, which introduced the modern image of Santa Claus as a man who delivers gifts on Christmas Eve with the help of his reindeer. Since then, reindeer have been associated with Christmas and the magical season.