Our plant pathologist looked at your photos and thinks this could be a bacterial infection, but we can't say for sure just by the photos. The symptoms are unusual. Christmas cactuses respond well to pruning. We recommend pruning out the symptomatic leaves. Disinfect your pruners with 10% bleach-water solution afterward. See if you get any new growth to come in without symptoms. Another option would be to take a few healthy leaves and try to propagate new plants from the leaf cuttings. There is no treatment we can recommend to reverse the leaf spots you see. 

Christmas cactuses should be watered regularly to keep the soil moist for most of the year, except for right after their flowering period when they prefer to be drier. In the winter, water enough to moisten the soil evenly but allow the top half-inch of soil to dry out before watering again. Resume plentiful watering when you see new growth in the spring. Never let the potting mixture dry out completely.

ckc