Certified GKH Gardening Expert
They can be quite specific. These plants require "seasons" while indoors, just as they would outdoors.
During the growing season, they will need 14 to 18 hours of bright, but indirect, light per day. It is important not to expose them to direct light to avoid burning the plant.
During the dormancy, they will need 12 hours or more of perfect, pitch-black darkness per day. They will also need to be watered MUCH less during this time. The temperatures should be around 50-55F (10-13C) during this time. Christmas cactus (If you are completely sure that it is, indeed, a Christmas cactus) will need about 4-6 weeks of this dormancy in order to flower. If this is an Easter, or Thanksgiving cactus, they will need longer, and shorter dormancies, respectively.
Once dormancy is over, you can return the plant to long days of over 14 hours and bring the temperatures back up.
This article will help you to care for the plant: https://www.gardeningknowhow.com/ornamental/cacti-succulents/christmas-cactus/advice-for-christmas-cactus-care.htm