Christmas is generally celebrated on December 25th.
However, that is not the case in many places around the world.
Christmas is recognized as a Christian holiday, and it is a day that Christians recognize the Birth of Christ. Within Christianity, there are different denominations and traditions that do things a bit differently.
This has to do with the calendar that they use.
Christmas Celebrations Across the World
Some jurisdictions of the Eastern Orthodox Church, including those of Russia, Georgia, Ukraine, Macedonia, Montenegro, Serbia, and Jerusalem, mark feasts using the older Julian calendar. As of 2021, there is a difference of 13 days between the Julian calendar and the modern Gregorian calendar, which is used internationally for most secular purposes.
As a result, December 25 on the Julian calendar currently corresponds to January 7 on the calendar used by most governments and people in everyday life. Therefore, the aforementioned Orthodox Christians mark December 25 (and thus Christmas) on the day that is internationally considered to be January 7.
However, other Orthodox Christians, such as those belonging to the jurisdictions of Constantinople, Bulgaria, Greece, Romania, Antioch, Alexandria, Albania, Cyprus, Finland, and the Orthodox Church in America, among others, began using the Revised Julian calendar in the early 20th century, which at present corresponds exactly to the Gregorian calendar. Therefore, these Orthodox Christians mark December 25 (and thus Christmas) on the same day that is internationally considered to be December 25, which is also the date of Christmas among Western Christians.
A further complication is added by the fact that the Armenian Apostolic Church continues the original ancient Eastern Christian practice of celebrating the birth of Christ not as a separate holiday but on the same day as the celebration of his baptism (Theophany), which is on January 6. This is a public holiday in Armenia, and it is held on the same day that is internationally considered to be January 6 because the Armenian Church in Armenia uses the Gregorian calendar.
However, there is also a small Armenian Patriarchate of Jerusalem, which maintains the traditional Armenian custom of celebrating the birth of Christ on the same day as Theophany (January 6) but uses the Julian calendar for the determination of that date. As a result, this church celebrates "Christmas" (more properly called Theophany) on the day that is considered January 19 on the Gregorian calendar in use by the majority of the world.
With the exception of the Armenian Patriarchate, everyone accepts December 25th as the date to celebrate Christmas, but the actual date depends on the calendar that is being used.