Today is Christmas on the Julian Calendar.

So, as a result, it's the day which the Orthodox who follow the Julian calendar, which is not all of them, celebrate Christmas.

Emblem of the Ukrainian Orthodox Church

In Ukraine, where the majority of Christians are in the Ukrainian Orthodox Church, which obtained autocephalous status on December 15, 2018, Metropolitan Epiphany, its head will lead a service in the Assumption Cathedral of the Kyiv-Pechersk Lavra monastery for the first time since 1685.  In that following year, the Ukrainian Orthodox Church fell under Moscow's authority. The Metropolitanate of Kyiv actually became an ordinary diocese of the Moscow Patriarchate in 1722 

This year, however, the Ukrainian Orthodox Church gave its members the option of celebrating Christmas on December 25, which became a widely discussed topic in Ukraine itself, where celebration of a civil Christmas on December 25 had already become widespread. The Ukrainian Greek Catholic Church, an Easter Rite Catholic Church which is the largest Eastern Rite church in the world, apparently already did.

The Ukrainian Orthodox Church's having obtained autocephalous status has been an odd backdrop to the war.  The Russian Orthodox Church is the largest Eastern Orthodox Church in the world, and it has been one of the primary opponents to reunion with Rome.  The relationship between the various Orthodox Churches is complicated on a legal basis, but generally the Ecumenical Patriarch of Constantinople is regarded as being the  primus inter pares between the various autocephalous church's heads, although sometimes the Pope is also referred to in that fashion.  He is regarded, generally, as having the power to accord autocephalous status, which at least from the outside is problematic as it would seem to suggest that he has a sort of superior authority which the Eastern Orthodox otherwise reject as to the Pope, even though they recognized early in their history.  Anyhow, the granting of autocephalous status by the Ecumenical Patriarch was fiercely resisted by Moscow, and it has lead to a round of schisms.  Moscow continues to deny that the Ukrainian Orthodox Church is autocephalous, and one of the claims of Russians in the war is that they are defending Orthodoxy.

As for the United States, about 1,200,000 Americans are reported as being in an Eastern Orthodox Church.  At least in Wyoming, most of the Eastern Orthodox Churches are Greek Orthodox, although they often have Russian Orthodox members and may be served by Priests who are from another branch of Orthodoxy.  Gillette has an Antiochean Orthodox Church, which represents a congregation which converted from Protestant fundamentalism following an intense study of the early church.