In your fictional world, what festivals, fetes and fairs are there? What are the seasons and are they marked in special ways? In my Fairy Kingdom, the villages compete to put on the best shows and the Fairy Queen and Eileen (prior to her defection) visited many. Sometimes that village rivalry is friendly, other times it is not (and has been a lingering source of grudges between places for years), but all of this I hope helps conjure up an image of what the background world of my novel is like. I hope it helps make that world seem more real and therefore believable.
What details can you give to your fictional world to make that seem real and believable? What can your readers identify with? What do the characters in your stories have in the way of cultural events and how do these affect the world you're writing about?
The whole idea of festivals, fetes and fairs was to give people a welcome break from the general drudgery of life. This was particularly true in medieval times. So what function does the cultural side of your fictional world have? Is it to keep the populace quiet (like the Romans did with their "bread and circuses")? And does anyone see through this and, if so, what do they do about it? Several story ideas there I think!