New Year's Eve or Old Year's Night is on the evening of December 31, the final day of the Gregorian year, and the night before New Year's Day.
New Year's Eve is a separate observance from the observance of New Year's Day. In modern Western practice, New Year's Eve is celebrated with parties and social gatherings spanning the transition of the year at midnight.
Many cultures use fireworks and other forms of noise making in part of the celebration.
Overhyped, overpriced, and not actually serving any real point or purpose other than marking the flip of the calendar. I've never really understood it as a holiday.