Today, January 7, in Ukraine we celebrate Orthodox Christmas day. Actually when I wrote "we" I meant not only Ukrainians but not all of orthodox Christians either. Even not all of Ukrainians. It sounds difficult but let's look on a short explanation.
Why the Christians have two Christmas at all? Actually three. But now let's be focused on two which is the widest celebrating.
A long time ago in the Galaxy far, far away... In 46 BC Julius Caesar did reform the Roman Calendar system and launched the calendar which later had been named on his honor. The Julian calendar was good but a little inaccurate so it was replaced in the 16th century by the Georgian calendar. The last one is more precise and we are using it in the present day.
As the Julian calendar is less correct it is lagging behind the Gregorian on 13 days and the gap is growing up. It means that January 7 by the Gregorian calendar is December 25, by the Julian right now. Because the part of Orthodox churches, including the Ukrainian Orthodox Church, still using the Julian calendar, many of holidays are displaced in time.
There are some historical reasons why the outdated calendar is still in use by churches. But what should we do now? Should we reform the church and move Christmas to December 25? There is no easy answer. At first, we shouldn't reform the church. In Ukraine, churches are separated from the state. And yes, in Ukraine is no monolith church that could be dominant in society. So all of the existing orthodox churches have to find a common decision by themself. And they will not. Cause there is another problem. Not only Christmas depends on a calendar. It is about all religion dates and many of them are important holidays which also will be moved in case of change the calendar system. It could be unacceptable for many people. So just let it be as it is. From 2017, December 25 is an official day off in Ukraine alongside with January 7. These days also are holidays in Belarus. So no matter which one of the calendars is using by the church, anyone can decide which date to celebrate by themself or don't celebrate any of them