Hi, Mitch. Enjoyed reading the myths associated with the seasonal plants!
Parrot Flower - Alstroemeria psittacina - grows and blooms well in my Zone 8b/9a garden, but in my climate, it blooms in June. A friend of mine always has a party on the weekend of Armed Forces Day. He has this plant as do I. I was out watering my plants one summer approaching dusk when I thought about the connection and suddenly realized I had a party to attend that night! I appreciate the little parrot flower every year for that reminder.
The myth about the poinsettia is interesting, but the "facts" (according to the professors in botany/horticulture at the University of Florida, a major poinsettia research school where newly introduced cultivars of poinsettia are tested and put up for sale), relating to how the poinsettia became associated with Christmas (at least in the U.S.) are that the principle grower of poinsettias in the early 1950's took advantage of the new fangled invention of television to make the public associate poinsettias with the holidays. The poinsettia growers supplied tons of poinsettias in flower as background scenery for such T.V. Christmas specials as Pat Boone's and Perry Como's holiday shows. The trick worked. The poinsettias became a fad that lasts until today. Less romantic than the tale you provided, but reality, unfortunately, is often crass.
The poinsettia having it roots in Mexico was part of Christmas long before TV, I am guessing that the growers saw this and brought it up here maybe? The plant was used - or so says the Mexican Gov. Official guides - by the Aztecs for winter holidays and was just carried over. Interesting stuff all the way around - love to find the roots of the plants I - sometimes fail but still try - to grow.