This is the 5913th most frequent Russian word.
рождественские
"Christmas" (adjective; related to Christmas).
The word 'рождественскими' is used as an adjective describing the type of garlands, indicating that they are related to Christmas.
Мы украсили дом рождественскими гирляндами.
We decorated the house with Christmas garlands.
In this sentence, 'рождественские' qualifies the stories as pertaining to Christmas, indicating their thematic nature.
Дети слушали рождественские сказки у камина.
The children listened to Christmas stories by the fireplace.
Here 'рождественском' is used in a locative sense, describing the market associated with Christmas.