Goose wrote:

. In rationalizing the celebration of Jesus’ birthday in late December, church leaders may have argued that since the world was allegedly created on the spring equinox (late March), so too would Jesus have been conceived by God on that date. The Virgin Mary, pregnant with the son of God, would hence have given birth to Jesus nine months later on the winter solstice.
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More likely, church leaders would have argued that since there was a tradition found throughout the pagan which held that great men and rulers were conceived on the day that would eventually be the same day as their death, it was necessary that the King of kings should be thus conceived.

The Synaxarion of the Orthodox Church says, concerning Good Friday, that this was "the twenty-fifth day of March"; yes, the day of the Annunciation to Mary and precisely nine months before December 25.