The delightful poinsettia that is ubiquitous during the holiday/Christmas season in the US has a fascinating history, starting from its origins in Mexico.
What we know as the poinsettia, the Aztecs called Cuitlaxochitl, and they had several uses for it including a dye from the red leaves and the milky sap (latex) that helps reduce fevers. Legend has it that Montezuma had them brought to Mexico city in big caravans since the plants couldn't be grown at such a high altitude.
Franciscan missionaries in the 17th century started using the plants for the Christmas procession called fiesta of Santa Pesebre which has a procession to a manger or nativity scene. There is also the legend of a very poor girl, often called Pepita, who is deeply sad that she has nothing significant to offer the baby Jesus. In some versions her cousin tells her that anything given in love is acceptable, in other versions it is an angel. She then gathers weeds by the road and when she offers them to the baby Jesus, they transform into poinsettias.
These vibrant plants caught the eye of the first American ambassador to the Mexico, Joel Roberts Poinsett. Poinsett's real love was botany and he sent numerous poinsettias back to his South Carolinian hothouses. (Poinsett was so fond of science that he eventually founded the National Institute for the Promotion of Science and the Useful, which eventually became the Smithsonian Institute.) He gave them to friends and other botanists, and they kept getting passed on until they reached a Pennsylvanian botanist Robert Buist who is the first person known to have started selling the plants.
And we have a National Poinsettia Day: December 12, in honor of the man who brought this plant to the US. Wonderfully, it coincides with the feast of Our Lady of Guadalupe, which is a huge celebration in Mexico and in which poinsettias are used everywhere.