This is the 5000th most frequent Russian word.
летних
"Летних" means "summer" (genitive plural or adjectival form related to summer).
"Летних" is describing vacations (каникул), showing the time period of summer.
Во время летних каникул мы поехали на море.
During the summer vacations, we went to the sea.
Here, "летних" is used as an adjective in genitive plural to describe gardens (садах) associated with summer.
Мы наслаждаемся прогулками в летних садах.
We enjoy walks in the summer gardens.
The word "летних" specifies evenings (вечеров) as being of the summer, indicating a type based on season.