This is the 3942nd most frequent Polish word.
sylwester
New Year's Eve.
Here, 'sylwester' is used as a noun referring to the holiday of New Year's Eve.
Sylwester to moje ulubione święto.
New Year's Eve is my favorite holiday.
In this sentence, 'sylwestra' appears in the accusative case of the noun, indicating the event being attended.
Wybieramy się na sylwestra do znajomych.
We're going to a New Year's Eve party at friends' place.
The word 'sylwestra' is in genitive case here, showing the context or time of the event.