Which calendar system determines the New Year's Eve date used by most countries today?

The Gregorian calendar determines the New Year's Eve date used by most countries today.

Under the Gregorian calendar, New Year's Eve falls on December 31 and New Year's Day follows on January 1. Pope Gregory XIII introduced the calendar in 1582 to correct the accumulated drift between the calendar date and the solar year.

The reform changed the rule for leap years: century years are not leap years unless they are divisible by 400. Thus 2000 was a leap year, while 1900 was not. This rule keeps the calendar more closely aligned with Earth's seasonal cycle than the older Julian system.

Countries adopted the Gregorian calendar at different times, and some religious communities continue to observe other calendars for festivals or New Year celebrations. Nevertheless, the civil date used for the internationally dominant January 1 New Year is Gregorian.