Originally, it was thought that short-day plants needed a short duration of daylight in order to flower. Now we know that flower formation is actually triggered by long periods of uninterrupted darkness. For poinsettia, at least 12 hours of each 24 must be uninterrupted dark. Night temperature also has an effect and should be below 70 degrees F with 60 to 65 degrees F preferred.
During the day, place the plants in the sunniest location of the house. This high level of light is needed for the plants to have the energy required for good bract coloration. Day temperatures should range between 65 and 75 degrees F.
Providing uninterrupted darkness can be a problem for gardeners unless there is a room in which the lights are never turned on. If you don't have such a room, place your poinsettia in a dark closet or cover it with a cardboard box each night for the required 12 hours. If using a cardboard box, tape all the seams with duct tape to cut off any light. Poinsettia takes anywhere between eight and 11 weeks to flower once the dark treatment has been started. Normally, people start the dark treatment in late September to early October. The first six weeks are critical as this triggers the plant to bloom. The remaining time is needed for flowers to develop. For every night you miss during the first six weeks, add two days to the bloom time.
After the six-week dark treatment, the buds have set and the dark treatment is no longer needed. (Ward Upham)