What calendar system makes New Year's Eve fall on December 31 for most countries today?

The calendar system that places New Year's Eve on December 31 for most countries today is the Gregorian calendar.

The Gregorian calendar was introduced by Pope Gregory XIII in 1582. It corrected a small but accumulating seasonal drift in the older Julian calendar by changing the leap-year rule. Under the Gregorian system, century years are not leap years unless they are divisible by 400.

The reform was adopted at different times around the world. Catholic countries generally adopted it first, while some Protestant and Orthodox countries retained the Julian calendar for much longer. Britain and its colonies changed in 1752, and Russia adopted the civil Gregorian calendar after the 1917 Revolution.

The Gregorian calendar is the civil standard in most countries, but other calendars remain important for religious and cultural observances. The Julian calendar is still used by some Eastern Orthodox churches for liturgical dates, so their Christmas and New Year observances may not align with the civil calendar.