The Old New Year (Russian: Старый Новый год) is a traditional holiday, celebrated as the start of the New Year (Новый год) by the Julian calendar (Юлианский календарь). In the 20th and 21st centuries, the Old New Year falls on January 13/14.
Although the Russian Soviet Federative Socialist Republic officially adopted the Gregorian calendar (Григорианский календарь) in 1918, the Russian Orthodox Church (Русская православная церковь) continued to use the Julian calendar. The New Year became a holiday which is celebrated by both calendars.
As in most countries which use the Gregorian calendar, New Year's Day in Russia is a public holiday celebrated on January 1. On that day, joyous entertainment, fireworks, elaborate and often large meals and other festivities are common.
The New Year by the Julian calendar is still informally observed, and the tradition of celebrating the coming of the New Year twice is widely enjoyed: January 1 (New New Year) and January 13 (Old New Year).
Usually not as festive as the New New Year, for many this is a nostalgic family holiday ending the New Year holiday cycle with traditional large meals.
via wiki