Being from Eastern Europe, so living in a country where the main religion is Orthodoxy, I returned to the root of the question, and the way to explain to West European colleagues why for me the tradition to meet Christmas day is on the 7th of January. How did it happen that in the world now there are two Christmas evenings, on the 24th of December and on the 6th of January? The short onservation - when the majority of the world meet this day in December, but these countries which are connected in a religious question with Russia they celebrate on the evening to the 7th of January.
Honestly, the answer lies in the differences in calendars. First, let's mention the solar year, it is set by astronomers that it is the amount of time that the Sun needs to return to exactly the same position in the following year, and measured as 365.2421897 days. Since it is more than 365 days, quarters of the day accumulate in one day once per 4 years, what is known as a leap year. Because otherwise, the calendar year would slowly go ahead of the solar year and the start of the spring could be at the end of June.
When the Romans noticed that the calendar is not consistent with dates because they wanted to have the same date for the day of the Winter and Summer solstices, as well as the March and September equinox, they decided to take an Egyptian calendar (Solar) and restructured it. This is the origin of the Julian calendar, named after Julius Caesar, who introduced it in the year 45 BCE, which was used by all Christian lands until 1582. Between 325, when the Julian calendar was firstly used by the church to define the date of Easter, and 1582 the solar calendar had drifted back for 10 days. So the spring equinox, which is the first day of spring, was on March 11 (whereas in 325 it had been on the 21st of March), the day which had the most daylight was the 11th of June and the shortest day was the 11th of December. Thus, some adjustments were requested.
After accepting Christianity in the 4th century, the Rome Empire and the Byzantine Empire were developing separately. The first initiated the Grigorian calendar under the rule of Pope Gregory XIII in 1582, with an idea that the winter solstice is on the 22d or 23d of December, after which Christmas day should be celebrated, and these two events should be connected. Accordingly, that was made in order to prevent the calendar used by the church from drifting any further from the seasons. He introduced an advancement where a century year (e.g. 1600, 1700, 1900, 2000, 2100) could only be a leap year if it was divisible by 400. From that, 1700, 1800, 1900 would not leap years, but 1600 and 2000 would be. Pope Gregory XIII also proposed that the calendar should be brought back in line with the seasons, according to which the spring equinox would again fall on March 21. This demanded that 10 days be omitted when moving from the former to the new calendar. Pope Gregory’s calendar, which is the one that nearly every country in the world uses today, is called the Gregorian calendar and was adopted by the Catholic countries in Europe in 1582, when the day of the year was moved forward, missing out for 10 days.
Nevertheless, the Protestant and Orthodox countries in Europe rejected to adopt the Gregorian calendar originally, feeling that it was a conspiracy by the Catholic church to impose its power over non-Catholic nations. This caused confusion over dates when some countries had converted to the Gregorian Calendar and other countries continued to use the Julian Calendar. Finally, in the 1750s, all the Protestant countries changed over to the Gregorian Calendar. While at the same time, the Eastern Christians did not switch and until now are following the Julian calendar for religious celebration.
The Orthodox countries of Eastern Europe were slower to adopt the Gregorian Calendar. The Soviet Union didn’t adopt it until the fall of the Russian Empire, after the revolution in 1918 and Greece didn’t adopt it until 1923. Of a big interest, though all the Orthodox countries have adopted it for civil intentions, most Orthodox churches still use the Julian Calendar and are not accepting a calendar that they perceive as standing imposed by the Catholic Church. For example in Ukraine, the country follows the Grigorian calendar, and even though the Ukrainian church is independent from a Russian Orthodox church, Ukrainians still meet Christmas in January according to the Julian calendar, which is later from the Gregorian one for 13 days, so is the 7th of January in the Gregorian Calendar.
If to say about the tradition, for the dinner in the holly evening, 12 dishes should be made that are oriented on a meatless diet. However, on the following day, that is Christmas, the fast is finished and an abundance of meat is served on the table.
Hopefully, we will never be confused and have conflicts because of Christmas day, will for sure respect both groups, and who knows, maybe one day when this holiday will be celebrated on the same day by all.