There are many popular explanations for why Christmas is on December 25. These are mostly modern revisionist attempts to explain away the significance of Christian holy days and generally relate to supposed attempts to suppress or appropriate earlier pagan practices. For example, Christmas was supposedly meant to replace Saturnalia, but this Roman bacchanal began on December 17 and never extended later than December 23. Another popular myth propagated by Bill Maher in his "mockumentary" Religulous is that attempts to suppress the cult of Sol Invictus was the reason for fixing the date, but this is hard to believe because the cult of Sol Invictus dates from the late third century, well after Christmas was fixed, and Sol Invictus was on the day of the solstice, which is not December 25.
The reason for Christmas on December 25 is the date of Easter. Methods used to calculate the date of Easter in the early Church varied; some continued to observe 14 Nisan as the date of Easter based on the Jewish Passover feast whether or not it fell on a Sunday; in parts of the west such as Gaul, the date of Easter was fixed on March 27, a practice that continued right up to the 6th century, and therefore Good Friday was March 25. If you are aware of the temporal cycle of the Church's calendar, you probably know March 25 as the day that the Solemnity of the Annunciation is observed, and this is not by accident; a common belied in the ancient world was that a great man's date of death coincided with that of his birth or conception. The Fathers of the western Church used this common belief when they established March 25 as the date of commemoration for both the conception and death of Jesus.
There was a different method of calculation used in the east, but despite that the dates agree and March 25 and December 25 are also observed in the Byzantine Churches (although the Orthodox retain the Julian Calendar which is now 12 days behind). The reasoning was more theological: it was held that the day of the new creation (Resurrection) corresponded to the date of the first creation, and these also shared the date of the Incarnation. The understanding was that there is an inextricable connection between all the mysteries of salvation, beginning with creation and including all the mysteries of Christ. Therefore it was natural to celebrate the new creation annually with its cardinal mysteries of the Incarnation and Resurrection. (See Ratzinger, Spirit of the Liturgy, 106).
It is obvious that December is nine months after March, but what I learned in Patristics class last semester is that the date of birth was figured from the date of conception, not the other way around as the importance assumed by the second holy day in contemporary public awareness might have led me to assume.
 |
| Resurrection and Annunciation, details from opposite doors of the Isenheim Altarpiece, Matthias Grünewald, 1515. |
This connection between the feasts was lost later when the calculation of Easter was standardized for the most part (the calculation long continued to vary, not being fixed by a council until Nicaea in 325, which also explains the ancient Armenian observance of the Nativity and Theophany on 6 January because of a different Easter dating). Another reason the connection is lost is the later change that when 25 March falls within Holy Week, the Solemnity of the Annunciation is transferred to the second Monday of Easter. This preserves it as a separate celebration, which only makes sense if it is seen separately.
The Fathers of the Church intended that the whole mystery of Christ from Incarnation to Pasch be celebrated together and that they be seen as inextricably connected. Although this connection has been lost in all but historical record, the two coincide (at least formally) a few times a century when Easter falls on 27 March*, a unique occasion, especially in this Jubilee of Mercy, to reflect on the unity of history in Christ who makes all things new.
*This actually happened twice recently, 2005 and 2016, and will not recur until year 2157.
