This is the 4148th most frequent Russian word.
рождеством
"Рождеством" means "Christmas" (instrumental case in Russian).
The word "рождеством" is used in the instrumental case to denote the event "Christmas" with the preposition "с" to signify a holiday celebration.
Поздравляю тебя с Рождеством.
I congratulate you on Christmas.
The word "рождеством" is used here as an instrumental subject to discuss the naming or association of the term "Christmas".
Рождеством обычно называют день рождения Иисуса Христа.
Christmas is usually called the birthday of Jesus Christ.
"рождеством" is used in the instrumental case implying the temporal or causal relation to the event "Christmas."