To seperate themselves from russia. As they use the Julian calander celebrateing in jan.
I assume it is also to unify with the west. As that has been aboided in the past due to russias threats.
But as theu no longer have reason to avoid joining nato. I imagine officially sharing the same calendar seem like a bery easy unifying move.
I aaaume anyone who feels the need to celebrate on the 7th jan still can
Although hard to understand why any obe would care. Its not like xmas is the actual date of any past event. But instead the time pagan religions in europe were already celibrateimg yuletide etc.