This is the 1573rd most frequent Polish word.
ferii
"Ferii" is the genitive and dative form of "ferie," which means "holidays" or "break" (usually school holidays).
In this sentence, 'ferii' is in the genitive case plural form, referring to a period of holidays.
Podczas ferii zimowych planujemy wyjechać w góry.
During the winter holidays, we plan to go to the mountains.
Here, 'ferii' is used as an object of time in a noun phrase, emphasizing the period of holidays.
Dzieci zawsze cieszą się na czas ferii.
Children always look forward to holiday time.
In this usage, 'ferii' appears as part of a prepositional phrase to indicate anticipation of the event.