This is the 12023rd most frequent Russian word.
зимних
"Зимних" means "winter" (adjective, plural form).
The word "зимних" describes the evenings' association with winter, and its form matches the plural and genitive case of "вечеров".
Зимних вечеров он часто проводил у камина.
During winter evenings, he often spent time by the fireplace.
Here "зимних" is used as an adjective modifying "ботинок", indicating that the boots are designed for winter use.
Я купил пару зимних ботинок на распродаже.
I bought a pair of winter boots at the sale.
In this sentence, "зимних" modifies "дней", highlighting the context of days during winter, using plural and genitive case.