Chuck Missler explained it this way.
Historical and Biblical evidence makes Jesus approximately 33 1/2 years old when He was crucified on the eve of the Passover in March-April.
That would put his birthday somewhere between mid August to mid-October.
Given this timing, it is entirely possible that his mother conceived Him in late December or early January, which is roughly congruent to the Advent celebrations of both Western and Eastern churches.
If you are inclined to do the work, Go back to 1 chron 24:10 to find the course of Abijah for the course Zacharias was working to see when Elizabeth got pregnant. add 9 months for John the Baptist to be born. Add 6 more for Jesus to be born. That puts Jesus right in the middle of Tabernacles. Keep in mind priests had to work their assigned course AND every priest worked 3 times a year on Passover, Pentecost, and Tabernacles. Going to a Jewish website like Chabad.org these things can be figured out.