From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-2068-452373
Question: How do I encourage Christmas cactus to bloom in December?
Mine always blooms in November.
Answer: Seems to be a common problem/complaint of Christmas cactus
owners. Perhaps it should be called simply a Holiday Cactus to cover
all bases. To encourage blooms on your Christmas cactus for the next
holiday season, there are a few things to keep in mind. First of all,
the plant needs a rest period (a period when growth slows or ceases).
To force it to rest, cut back on moisture so the soil stays just
barely moist. Keep the plant in a spot where the temperature never
exceeds 65F (18C) and allow only ten hours of light each day. As a
native of South America, Christmas cactus is a short-day plant
requiring about fourteen hours of complete darkness each day to
encourage bud development. From the onset of buds it takes about 12
weeks for blooms to appear.