Is the date of our annual celebration of the Nativity supposed to be seen as a dogmatic statement of a historical date?
Does the Church teach that Christ was born on December 25th?
Does the Church celebrate Christ's birth on some other date?
Just because something is not enshrined in dogma, but is hallowed by tradition, does that mean that it can be tossed aside? It may not be dogma, but the Church bids us celebrate the Nativity of Christ on that date, believing that that is the correct day, not because of history, per se, but because it is the day of liturgical celebration. Same thing with the Transfiguration. Aug. 6 has always been the day, even though the Lord, according to the Gospel, was transfigured shortly before His crucifixion. But Aug. 6 is the date, because it is 40 days before the Exaltation of the Holy Cross, on Sept. 14.