A Christmas cactus likes to be watered when the soil starts to get dry. Feel the soil. If it feels dry, it's time to water. Don't over-water it but don't under-water it either. They like bright but indirect light. If your growing atmosphere is dry, provide a source of humidity such as a tray of water next to the plant so that the water evaporates and provides humidity. Or you can make a humidity tray by placing the pot on a waterproof saucer that is filled with gravel and filled halfway with water.
These plants are beautiful in bloom but a lot of people have problems getting them to bloom each year. Here is a guide to help with that:
http://garden.org/ideas/view/S...