Answer: You can repot your christmas cactus when it has finished blooming. (If you repot while it is in bloom it may drop its flowers.) You'll want to choose a pot that is just one size larger than the current pot (they bloom best when slightly pot-bound), and use fresh potting soil to fill in the empty spaces after you've moved it into the new pot. For excellent drainage you can choose cactus mix potting soil but I've had good luck with just regular potting soil. Once you've replanted, water it well to help settle the soil.
Best wishes with your plant!
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