Orthodox Christians and Greek Catholics mark Christmas Eve on Monday, January 6.

The Nativity Fast, which lasts from November 28, ends on January 6. The fast was established in a period of early Christianity so that Orthodox Christians could be cleansed of their sins through penance, prayer and abstinence before Christmas.

The Nativity Fast ends when it gets dark or after the first star is seen in the evening sky, which symbolizes the Star of Bethlehem at the time of Jesus' birth. According to the Gospel of Matthew, after seeing the star, wise men from the east came to worship the Christ and brought their gifts to him.

Christmas is one of the twelve great feasts and is marked on December 25. Churches that use the Julian calendar celebrate Christmas on January 7 according to the Gregorian calendar (in the 20th-21st centuries).

The Ukrainian Orthodox Churches, Russian, Jerusalem, Serbian, Georgian Orthodox Churches and the monasteries of Mount Athos, as well as the Ukrainian Greek Catholic Church and several other Eastern Catholic Churches, celebrate Christmas on December 25 according to the Julian calendar, which corresponds to January 7 according to the modern Gregorian calendar.