This is the 9032nd most frequent Russian word.
рождественскую
"Рождественскую" means "Christmas" (adjective form, feminine singular or agreeing with feminine singular nouns).
In this sentence, 'рождественскую' is the accusative form of the adjective 'рождественский', describing the noun 'елку' (tree) to specify that it's a Christmas tree.
Мы украшаем рождественскую елку.
We are decorating the Christmas tree.
Here, 'рождественскую' again appears in accusative form and modifies 'сумку' (bag), indicating its relation to Christmas.
Она купила рождественскую сумку для подарков.
She bought a Christmas bag for gifts.
In this instance, 'рождественскую' describes 'скидку' (discount) and provides a temporal and thematic context of the discount connected to Christmas.