What does the “Eve” in “New Year's Eve” traditionally refer to: which date?

The “Eve” in “New Year’s Eve” refers to December 31, the day before New Year’s Day.

New Year’s Eve is the final day of the Gregorian calendar year. Celebrations commonly begin during the evening of December 31 and continue through midnight, when the calendar changes to January 1. The exact local date and time of the transition depend on the time zone.

The word “eve” generally means the evening or day preceding a festival or important event. In this case, it identifies the night before New Year’s Day rather than the first day of the new year itself. Some cultures observe additional customs throughout December 31, while others begin New Year observances after midnight.

Not every calendar celebrates its new year on January 1. The Gregorian calendar is the civil calendar used internationally, but religious and cultural calendars may have different New Year dates. Thus December 31 is specifically New Year’s Eve in the Gregorian-calendar context used by most international civil schedules.