Watering Christmas cacti depends on several factors, including the potting medium, the size and growth stage of the plant, the humidity level in your home, and the time of year. Generally, water them only when the top inch of soil is dry. If the plant is in a small pot, it may require more frequent watering. In the winter, when the plant is dormant, you can reduce watering to every two to four weeks. Always use room temperature water and avoid overwatering, which can lead to root rot.