Which calendar system determines the January 1 date used for most modern New Year's Eve celebrations?

The Gregorian calendar determines the January 1 date used for most modern New Year's Eve celebrations.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the Julian calendar's accumulated drift against the solar year. The reform changed the rule for leap years and initially skipped several calendar dates, bringing the calendar year back into closer alignment with the seasons.

Adoption was gradual rather than universal. Catholic countries generally adopted it first, while many Protestant and Orthodox societies continued using the Julian calendar for some time. The Gregorian system is now the civil calendar used internationally, although religious and cultural calendars remain important in many communities.

January 1 is therefore a civil-calendar convention, not a date shared by every New Year tradition. Lunar, lunisolar, and religious New Year festivals can fall on different dates. Even countries using the Gregorian calendar may celebrate additional New Year observances according to local custom.