The Julian calendar makes New Year’s Eve fall on January 13 for communities observing Old New Year.
The Julian calendar was introduced by Julius Caesar in 46 BC and took effect in 45 BC. It slightly overestimates the length of the solar year, so its dates gradually drift relative to the Gregorian calendar adopted in much of the world.
In the twenty-first century, the Julian calendar is 13 days behind the Gregorian calendar. Consequently, January 1 in the Julian calendar corresponds to January 14 in the Gregorian calendar, making the preceding evening January 13 in Gregorian dating.
Not every Eastern Orthodox community uses the old calendar for civil New Year observance, and religious calendars can differ by church. “Old New Year” is therefore a cultural and calendrical term, not a universal rule for all Orthodox Christians.