My understanding that the December 25th date was choosen because it’s symbolic, not because it was believed to be the actual date of birth. The symbolism was that the 25th is the 3rd day after the winter solcest(sp), the lowest point of the suns orbit in the southern hemisphere in a given year. Thus, just as Christ rose from the dead three days after his execution, the birth of Jesus is set on the 3rd day of the rising sun.
I would say given when the 'course' of Abia takes place mid June on our modern calendar, that the conception is marked for purpose, to us in Christianity as point of reminder that on the shortest day of literal sunlight the giver of life joined us in the flesh.
Actually at that time the winter solstice fell on December 25. Roman pagans celebrated Dec. 25 as the birthday of the sun because that’s when the sun began to move north in the sky. Because the Julian calendar was slightly off (by one day every 128 years) the actual solstice began to fall earlier. The reason the solstices and equinoxes now fall about the 21st of June, December, March and September is that Pope Gregory XIII wanted to get the calendar back to where it was in A.D. 325, because the Council of Nicaea had laid out the rules for calculating the date of Easter.