This is the 4552nd most frequent Polish word.
letni
"Letni" in English means "summer" or "summery." It can also mean "lukewarm" depending on the context.
The word 'letni' is used here as an adjective to describe the season 'summer,' linking it to the day described.
To jest letni dzień pełen słońca.
This is a summer day full of sunshine.
'Letni' here describes the temperature of the tea, indicative of it being moderately warm.
Ta herbata jest letnia, niezbyt gorąca ani zimna.
This tea is lukewarm, neither too hot nor cold.
In this sentence, 'letni' characterizes the concert as related to or happening in summer, providing temporal context.