Who would they commemorate and why? Are such celebrations held by all or are there those opposed to the festivals (and if so, why?)? What would be holy to your fictional world?
Would your world's government make it compulsory to celebrate the festivals or would they try and ban them? What is the history behind the festivals and why your fictional world adopted them? Is there trouble at these celebrations (alcohol induced or not) or do they always go well? Are celebrations like this a highlight of your fictional world's year or do people groan when it is time to get the bunting and other decorations out again?
There must be several story ideas here!