A beautiful holiday plant, the Christmas Cactus (botanically known as Schlumbergera or Zygocactus) blooms at Christmas time if cared for properly. A month prior to Christmas, you will be able to watch the tips of the leaves beginning to grow. The tips grow darker as each day passes, until a bud forms. At Christmas time, as if by magic, the buds open to a beautiful flower that will add color and warmth to any holiday season.
Purchase a Christmas Cactus. Before October, you can find them without any flowers buds. From late October through December, they are usually available with buds or already blooming. Most of them will be in pots that are no more than 8 inches in diameter, although plants in bigger pots are probably left over from the previous year and are more desirable because they're more established. As they are a fickle plant, a more established plant will likely be more able to cope if the environment is not perfect.
Keep your Christmas Cactus in appropriate conditions. A Christmas Cactus will appreciate bright but indirect light. Keep the plant in a well-lit location (like a near a window or under a skylight) away from direct sunlight (which can stunt growth and burn the leaves), drafts, heat vents, fireplaces or other sources of hot air. Move an indoors plant outdoors in summer to a shady location. It is best to keep in a normal house temperature range (65 to 80 degrees F, 18 to 27 degrees C).
Provide a source of humidity if you live in a dry environment. Put a glass, vase or tray of water next to the plant so that the water evaporates and provides humidity. Alternatively, you can make a humidity tray by placing the pot on a waterproof saucer that is filled with gravel and halfway filled with water.
Water a Christmas Cactus with care. Caring for this plant can be a bit tricky as you need to take care not to overwater or underwater it:
Care must be taken not to underwater it, as a Christmas cactus is in origin a tropical plant, not a true cactus. Unlike many cacti, this variety cannot tolerate completely dry soil. If the soil gets too dry, the flowers buds will drop, and the plant will wilt. Feel the soil with your fingers; if it feels dry, it's time to water.
Too much watering will cause spots from white rot to appear on the leaves, and the leaves will likely fall off. The soil should be evenly moist for best growth. The rule of thumb is, less water is better than too much water.
When watering, thoroughly water the plant. Before attempting to water the plant again, check to see that the top inch of soil has dried thoroughly first. Mist leaves as well as watering the soil.
Water the cactus based on your environment and the time of year. A good method is to water a cactus as follows:
Dry climate, outdoors: Water every two to three days when warm and sunny.
Humid, cool or indoors: Water every week.
During the fall and winter months, the plants should be watered less frequently in order to promote blooming.
Add fertilizer to assist plant growth. Plants that are actively growing should be given a blooming houseplant-type fertilizer. Follow the label directions for how much and how often to feed. Fertilizing is important to keep the plant in good condition; the joints are fragile and can break apart if the plant descends into poor health. Generally, it should be fertilized 2-4 times a year with a 20-20-20 feed, but stop feeding about a month before the buds appear (this usually means stopping by the end of October).
Encourage the flower blooming for the holiday season. The key to getting Christmas cactus to flower during the holiday season is proper light exposure, correct temperatures and limited watering.
Because this plant is thermo-photoperiodic, it will set buds when day length is about equal to night length and when the temperature drops to 50 to 60 degrees F (10 to 15 degrees C) for several weeks. 
During the fall months, the Christmas Cactus should be placed in a spot where it receives indoor indirect, bright light during the daylight hours but total darkness at night (absolutely no artificial light). The Christmas Cactus requires long, uninterrupted dark periods, about 12 or more hours each night. Begin the dark treatments in about mid-October to have plants in full bloom by the holidays. Place the plants in a dark area from about 12 or more hours each night for 6-8 weeks or until you see buds forming. A closet or unused bathroom are ideal places.
From September and October, the Christmas Cactus should be kept in a cool room where temperatures will remain around 50 degrees, give or take a few degrees. Don't expose the plant to freezing temperatures. Plants should be blooming for the holidays if cool treatments are started by early November.
Be especially careful with watering at this time. Reduce the watering slightly. Do not soak the soil after a dry period; only moisten the top few inches, since buds, flowers and even leaves can fall off if the roots are suddenly saturated.
Prune the Christmas cactus about a month after blooming. This will encourage the plant to branch out, especially after a period of "rest" has been granted. It will not look very pretty after the blossoms have faded. Some people wait until March or so, when new growth begins, to prune the cactus.
Propagate a Christmas Cactus by cutting off sections of the stem. Each section must consist of two or three joined segments. Allow each section to dry for a few hours before pushing them into a 3-inch pot that contains the same potting soil as the parent plant. Plant halfway down the first segment. Treat the cuttings as mature plants. In about four to six weeks, the cuttings should have rooted and will begin to show some new growth. They grow quickly, and should take in about two or three weeks. You can fertilize after cutting has grown one new segment.
The blooming instructions are based on having a cold Christmas. If you reside in the Southern Hemisphere, you may have more difficulty finding this plant around Christmas time, and they'll need more controlled conditions in order to bloom during the holidays. You'll probably have better luck getting them to bloom during the coldest months of the year.
The best time for repotting is between February and April, about once every three years. Just be sure not to give it a pot that's too big; the Christmas Cactus flowers best when it's pot-bound. Provide a soil that's well-drained with good aeration (e.g. one part potting soil, two parts peat moss or compost, and one part sharp sand or perlite).
Christmas Cactus is a beautiful plant that can bloom year after year; you will want to keep it even after the holidays pass. You can even pass it down through generations.
One of the most frustrating things that can happen to Christmas cactus is after the flower buds have developed they drop off the plant. Bud drop can be caused by several different conditions. Usually it's because of over-watering, lack of humidity or insufficient light.
Any sudden changes in temperature, light and watering will damage the Christmas Cactus. Drafts and temperature extremes can cause the flower buds to drop from the plant before they have a chance to open. Introduce changes gradually.
The Christmas cactus should never be placed near a door that opens and closes to the outside. Likewise, keep it away from heating ducts or near the fireplace or drafty areas.