Which calendar establishes January 1 as New Year's Day in most modern countries?

The Gregorian calendar establishes January 1 as New Year's Day in most modern countries.

Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct a gradual drift between the calendar year and the solar year. The reform adjusted the leap-year rule and removed several accumulated calendar days. January 1 remained the first day of the year in the reformed system.

Countries adopted the calendar at different times. Catholic states generally adopted it first, while Britain and its colonies switched in 1752 and Russia adopted it after the 1917 Revolution. Today it is the civil calendar used internationally, although many communities also observe religious or traditional calendars.

The Julian calendar also uses January 1 as New Year's Day in its own tradition, so the calendar's role should not be confused with inventing the date. The Gregorian system is significant because it became the dominant civil standard worldwide.