So when did poinsettias become a "thing" at Christmas? I looked it up on National Geographic's website. Here's what I found:
"Franciscan missionaries arrived in Mexico in the 16th century and eventually began setting up elaborate manger scenes at Christmastime. Holly, Europe’s holiday flora of choice, was nowhere to be found for the dioramas, but when the missionaries saw the red and green colors of this local plant—that happened to burst into color every December—they knew they had the perfect stand-in."