We all know it wasn't December. The WT has always said the first part of October. The reasoning I remember is that they calculate that Jesus' ministry was 3-1/2 years long; and since Jesus died in the spring of 33 CE, he must have been baptized in the fall of 29 CE. Since he turned 30 years old when he was baptized, he must have been born in the fall.
On TV the other day, I heard a commentator say that Jesus was born in the spring, but he offered no proof, other than that the shepherds were in the field at night. Clyde commented that would make sense because the spring is when the sheep would have their young lambs
Two questions: 1) How did the WT calculate the 3-1/2 years? Did it have anything to do with Daniel's prophecy of the 70 weeks of years? - 2) Does the Bible say Jesus was exactly 30 years old when he was baptized?
And, really...it doesn't make any difference.