This is the 9033rd most frequent Russian word.
рождеству
"To Christmas" (dative case of "Рождество").
Here, 'Рождеству' is in the dative case, implying the action of being directed toward or related to Christmas.
Мы готовимся к Рождеству.
We are preparing for Christmas.
In this sentence, 'рождеству' is used in the dative case to indicate the time by which the action took place.
К рождеству он купил подарок.
By Christmas, he bought a gift.
Here, 'Рождеству' is in the dative case as the object of the verb 'радуются,' which often requires the dative case.