It should be noted that many Orthodox churches have actually been using 25th December as the date for 100 years now, following the 1923 Council of Constantinople. The choice to shift to the Gregorian date (or rather the Revised Julian date, but it's the same day as the Gregorian one) is just a shift to align with other Eastern Orthodox patriarchates rather than a departure from Eastern Orthodoxy altogether

All this just proves that Christians don't know the first thing about astronomy because the actual date of the solstice flips between the 21st and 22nd of December. Which should actually be the 31st and 1st according to the original intent of the calendar, as said, Christians have no clue about Astronomy and allowed the thing to drift.