In my country there are quite a few, I would say that the most famous are, San Fermines, the Fallas, Holy Week and Tomatina among some others, which are the most famous cultural festivals in your country, what is special about them?

Tomatina: People throw tomatoes and everyone ends up soaked in tomato sauce.
Holy week: Many will know it, thrones of saints are walked around the city and reverence is made to them.
San fermines: Bullfight throughout the city.
Fallas: Dolls made of wood and other materials are burned to attract good luck.