About half a century ago (in my teens) I was a keen member of the East Anglian Cactus Society, learning how to how cacti from seed and to nurture them into flowering. My distant memory tells me that a Christmas Cactus (like many other cacti) needs maximum sunlight, as long as it's not actually scorching. It also tells me that, like all cacti and succulents, it needs a period of rest (with minimal or no watering) for several months. (That's where most people go wrong with cacti; they carry on watering them all year round. Some shouldn't be watered at all for periods of 4 months, or even longer)
However, after that, I'm happy to let the RHS fill in the gaps ;-)
https://www.rhs.org.uk/advice/profile?PID=840