This is the 7628th most frequent Polish word.
letniego
"Letniego" means "summer" or "of summer" (genitive singular or accusative singular masculine/neuter form in Polish).
The word 'letniego' modifies 'spaceru' (walk), emphasizing that it took place in summer.
Podczas letniego spaceru podziwialiśmy piękno przyrody.
During a summer walk, we admired the beauty of nature.
Here, 'letniego' is an adjective describing 'dnia' (day), specifying the season during which the day occurred.
Letniego dnia upał był niemal nie do zniesienia.
On a summer day, the heat was almost unbearable.
The word 'letniego' is used as an adjective describing 'wieczoru' (evening), indicating that it occurs in summer.