Le réveillon is the evening before Christmas (Christmas Eve) and the evening before New Year’s Day (New Year’s Eve).  In both instances the evening is a celebration involving a gathering of family and/or friends and a festive meal. The term derives from the word réveil meaning "waking up," as in both cases it’s necessary to stay awake late into the night to celebrate.

The practice is followed in Greece, too. We even use the same French word!

The term is first documented in 18th century France, and was used by the French as a name for the night-long dinner parties held by the nobility.