This is the 871st most frequent Polish word.
festiwalu
The word "festiwalu" is the genitive or locative singular form of "festiwal," which means "festival" in English.
'Festiwalu' is used here in the locative case to indicate the location of an action (being at the festival).
Byłem na festiwalu muzycznym w zeszłym tygodniu.
I was at a music festival last week.
'Festiwalu' appears in the genitive case, describing possession or association with 'atmosfera' (the atmosphere of the festival).
Atmosfera festiwalu była niezwykle radosna i pełna energii.
The atmosphere of the festival was extremely joyful and full of energy.
'Festiwalu' is used in the genitive case here as part of a complex sentence, denoting the subject of the description ('events of the festival').