How do I care for a Christmas cactus?

Basically, care for it like any other plant—a bright location, water when the soil approaches dryness (despite the name it is not a desert plant), cooler temperatures away from heat vents and so on will help the blooms last longer.

Christmas cactus is a tropical plant that requires a highly organic soil mixture and lots of moisture. The plant likes bright light and average household temperatures. You can take your Christmas cactus outdoors during the summer and fertilize it monthly with a diluted houseplant food. Then bring it back indoors at the end of September and provide total darkness for 16 hours each day so the plant can set flower buds. The easiest way to accomplish this is to place it in a bright room for 8 hours and then either put a box over it or put it in a closet for 16 hours. It needs absolute darkness; even a short burst of daylight will retard the formation of flower buds.

During this bud-forcing period, keep the temperature between 60°F and 70°F, and do not fertilize the plant. In early December your Christmas cactus can be brought into ordinary light and will bloom in a few weeks.