The Julian calendar is the calendar system behind January 13 New Year’s Eve for Old New Year observers.
In the 20th and 21st centuries, the Julian calendar is 13 days behind the Gregorian calendar used for most civil life. As a result, December 31 on the Julian calendar corresponds to January 13 on the Gregorian calendar. The following day, January 1 Julian, is observed as Old New Year on January 14 Gregorian.
Old New Year is celebrated in parts of Eastern Europe, the Balkans, the Caucasus and elsewhere, often alongside religious traditions that retain the older calendar. The date is not a separate calendar holiday created by the Gregorian system; it is the civil-date equivalent of a New Year celebration calculated under the Julian system.
The Julian calendar was introduced under Julius Caesar in 45 BCE and used a leap-year rule that was slightly too long compared with the solar year. Over centuries, that small error accumulated. Pope Gregory XIII’s calendar reform corrected the drift, producing the Gregorian calendar and the 13-day difference now associated with Old New Year. The gap was 12 days before 1900 and will change again after 2100.