Pepita, a poor Mexican girl who had no gift to present the Christ Child at Christmas Eve services. As Pepita walked sorrowfully to church, her cousin Pedro tried to console her. “Pepita,” he said, “I am certain that even the most humble gift, given in love, will be acceptable in His eyes.” Pepita gathered a bouquet of common weeds from the roadside, for this was the only gift she could give. As she entered the chapel and approached the alter, her spirits lifted. Forgetting the humbleness of her gift, the girl laid the weeds at the feet of the Christ Child. Suddenly, Pepita’s ordinary weeds burst into brilliant red blooms! This miraculous event was named the Flores de Noche Buena, or Flowers of the Holy Night. Today, we call these flowers poinsettias.
Dr. Joel Poinsett, who was the first ambassador to Mexico, brought the bright red star-shaped flower to the United States. Hence, it was names as Poinsettia.
A very popular misconception is that poinsettias are poisonous, but scientific tests have shown that this is false. It is true, though, that the milky latex sap can be irritating to some, particularly individuals with sensitive skin, but the plant in general, is safe.
Reference: http://lacey.patch.com/blog_posts/the-legend-of-the-poinsettia