The Gregorian calendar makes January 1 the first day of the year in most countries, with New Year's Eve falling on December 31.
Pope Gregory XIII introduced the calendar in 1582 to correct a small but accumulating drift in the older Julian calendar. The Julian system had made the average year slightly too long, gradually moving calendar dates away from the seasons. The Gregorian reform adjusted leap-year rules and initially removed several dates from the calendar.
Catholic countries adopted it first, while many Protestant and Orthodox countries switched later. Britain and its colonies adopted it in 1752, and Russia adopted it after the 1917 Revolution. Some religious traditions and countries still use other calendars alongside it.
The Gregorian calendar is a solar calendar, but its New Year’s Day is a civil convention rather than an astronomical event. The December solstice, for example, does not occur on a fixed calendar date worldwide.