This is the 17982nd most frequent Russian word.
фестивальную
"Фестивальную" means "festival-related" or "pertaining to a festival."
In this sentence, 'фестивальную' functions as an adjective, modifying the noun 'площадь' (Square), and it describes a specific type of square associated with festivals.
Мы посетили фестивальную площадь города.
We visited the Festival Square of the city.
Here, 'фестивальную' again serves as an adjective, modifying the word 'программа' (Program), indicating that the program pertains to a festival context.
Организаторы подготовили фестивальную программу.
The organizers prepared a festival program.
'Фестивальную' in this sentence is used adjectivally to describe 'коллекцию' (Collection), suggesting clothing designed for or inspired by the festival aesthetic.