Which calendar system makes New Year's Eve the last day of the year on December 31?

The Gregorian calendar makes December 31 the last day of the year and New Year’s Eve.

In the Gregorian system, the calendar year runs from January 1 through December 31. New Year’s Eve therefore falls on December 31, with the new year beginning at midnight and continuing as January 1. The date is used by most countries for civil and international scheduling, although other calendars remain important for religious and cultural observances.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the Julian calendar’s gradual drift against the solar year. The reform adjusted the leap-year rule: century years are not leap years unless divisible by 400. This keeps the calendar more closely aligned with Earth’s orbit.

The Julian calendar also uses December 31 as its final date in a calendar year, so it is a tempting distractor. However, the question’s standard modern convention refers to the Gregorian calendar. Islamic and Hebrew calendars use different month structures and do not place their New Year’s Eve on December 31.