Why do Russians celebrate Christmas 7th of January? A difference happened because of using different calendar style. Our Russian Orthodox Church and few others use “old” Julian calendar, but majority other Churches use “new” Gregorian calendar, which was introduced the use in 1582 by Pope Gregory 8. It was done in order to correct an increasing difference between an astronomical year and calendar year, which was in Julian calendar. This calendar wasn’t precise enough. But Russian Orthodox Church didn't adopt it. The difference between calendars increases every 100 years by a day. At present it is 13 days. I have to add that would be better to say our Church celebrate Christmas 25 of December but of another calendar. We don't use it in an ordinary life.