proper noun/sɛ̃.sil.vɛstʁ/
New Year's Eve
“Chez nous, à la Saint-Sylvestre, la porte est toujours grande ouverte ! At our house, on New Year's Eve, the door is always wide open!”
Due to the Gregorian calendar reform of 1582, whereby the feast day of Pope Sylvester I came to fall on the 31st of December.