So what customs exist in your fictional world? Does it acknowledge Christmas or have its own festivals? What are these festivals based on?
Does everyone follow these customs? If not, what do the majority make of those who don't? Is there true religious (and other) freedom or are people punished for not joining in with what is accepted behaviour?
In a fictional world which links to others, what does your main world make of the customs to the other world it is connected with? Individually, how do your characters celebrate Christmas and/or other festivals? Are there customs your world wishes others would abandon or adopt and does it try to force the issue?
Definitely some story ideas there!