Plan to repot Christmas cactus only about once every three years. This will keep the plant slightly root bound, as the Christmas cactus prefers, without causing undue stress. If you're not sure when it was repotted last, you can let the plant tell you when it needs a larger pot. A Christmas cactus that wilts within a few days of watering likely needs repotting.
Size Up Slowly
Use a container no more than 1 inch larger than the current container when you repot a Christmas cactus. Moving any plant from a small container to a much larger one can cause the soil to hold too much water, leading to rot problems. Christmas cactus, being both sensitive to overwatering and preferring a slightly root bound growing environment is particularly susceptible to wet soil in a large pot.
Any pot you use should have drainage holes. The holes allow excess water to run out, minimizing root rot problems. To protect surfaces in your home, set the pot on a saucer and empty out any water that collects in it after watering. Plastic, metal, wood or glazed or unglazed clay containers work well but terra cotta, or unglazed clay is best as it allows air circulation around the roots.
Soil and Season
Repot Christmas cactus in the spring after the winter blooming season. Use well-draining cacti and succulent potting soil. If you don't have cactus soil available, you can use a general-purpose potting soil mixed with an amendment to improve drainage, such as 2 parts potting soil to 1 part vermiculite or sand.
To repot, pull the Christmas cactus out of its container. Inspect the roots and cut off any that look mushy. Add 1 to 2 inches of potting mix to the bottom of the container and then set the root ball into the pot. Adjust the soil in the bottom until the base of the stem sits level with the lip of the pot. Fill in around the sides with more potting mix. Set the pot in a sink -- or outside if it's warm -- and water until moisture drains out the hole in the bottom of the pot.