Why do poinsettias bloom in December?

Because the month falls within the range of 65-85 days after the initiation of the flowering process December is the time when poinsettias (Euphorbia pulcherrima) can be expected to bloom.

Specifically, bloom time results from the interaction of a plant's internal biochemistry with set environmental conditions. In the case of poinsettias, the natural timing of that initiation falls in September and October. But in fact, that flowering has the possibility of starting up in poinsettias any time the plant undergoes 12 hours 20 minutes of uninterrupted darkness for five (5) days straight ... if such other conditions as air and soil moisture, light intensity and temperature are met.