The Nahuatl (Aztec) name for this plant is Cuetlaxochitl, which means "flower that withers" and it was believed to represent the new life that warriors attain in battle. The Poinsettia gets its name in English from the first U.S. Ambassador to Mexico, Joel Roberts Poinsett, who brought the first samples of the plant to the United States.
There is a legend surrounding the Poinsettia. A poor peasant girl was going to mass on Christmas Eve ("Noche Buena" in Spanish, which literally means a "good night"). She was very sad because she didn't have a gift to present to baby Jesus. On her way to the church she gathered a few plants to take with her. When she arrived at the church she offered the plants she carried and realized that they had turned from green to bright red.
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