Here's why:
* Common nouns refer to general, non-specific people, places, or things.
* "Festival" is a general term for any event that celebrates something, and it can refer to many different types of festivals (music, film, food, etc.).
Examples:
* I went to a festival last weekend. (general festival)
* The festival featured live music, food vendors, and art displays. (still referring to a general festival)
Let me know if you have other words you'd like to classify!