What customs would your fictional characters keep for special festivals and feast days? What are these festivals and feast days and why are they celebrated? Is there anything in common with those we hold on Earth?
What foods and drinks go with these customs? If there are sacrifices, what type are they? What happens to those who don't celebrate? Do your characters decorate their homes for these special events and if so with what?
If you have a fictional world that knows about other civilisations (including perhaps our own), what does it make of what it observes?