So what does your fictional world(s) have in the way of high days and holidays? Are these celebrated by all? Is it compulsory to celebrate and what happens to those who refuse to do so?
Within your world, what countries exist? What are their unique celebrations? What triggered the commemorations being an important part of the culture you've set up? Each commemoration should have its own history which the residents of your worlds will know but where you may need to drop a line or two so your readers know what these events are all about.
How are holidays celebrated? Is there bunting and food or are more sombre means used? And how many holidays are held in a year? Do people/beings of choice look forward to these events or dread them for the disruption?