This is the 1294th most frequent Polish word.
kwietniu
"Kwietniu" is the locative or dative form of "kwiecień," meaning "April" in English.
The word "kwietniu" is used here as the locative case of "kwiecień" (April), paired with the preposition "w" to denote time.
W kwietniu zaczyna się wiosna.
In April, spring begins.
Here, "kwietniu" again represents the locative case of "kwiecień" after the preposition "w", specifying the time for the trip.
Planowana wycieczka odbędzie się dopiero w kwietniu.
The planned trip will take place only in April.
Similar to previous examples, "kwietniu" is in the locative case following "w" to indicate the time of birth in the month of April.