The frequency of watering the Christmas cactus plant will depend on various factors, including the size and type of the Christmas cactus, the climate in which the Christmas cactus is grown, and the humidity level. Generally, the Christmas cactus should be watered when the soil has begun to dry out and is still slightly damp. the Christmas cactus plant should not be completely dry, as this can lead to it dying or dropping its flowers prematurely.