[PapaBear3625]

One thing: the Catholic Encyclopedia says the date of the Annunciation was calculated from Christmas, not the other way around:

The year and day of the Annunciation cannot be determined as long as new material does not throw more light on the subject. The present date of the feast (25 March) depends upon the date of the older feast of Christmas.

[Tax-chick]

Obviously there are conflicting views among historians and their sources. This is hardly unique to the question of the date of the Nativity or its celebration.