Luke 1 says Zechariah was performing priestly duty in the Temple when an angel told his wife Elizabeth she would bear John the Baptist. During the sixth month of Elizabeth's pregnancy, Mary learned about her conception of Jesus and visited Elizabeth "with haste."The 24 classes of Jewish priests served one week in the Temple, and Zechariah was in the eighth class. Rabbinical tradition fixed the class on duty when the Temple was destroyed in A.D. 70 and, calculating backward from that, Zechariah's class would have been serving Oct. 2-9 in 5 B.C. So Mary's conception visit six months later might have occurred the following March and Jesus' birth nine months afterward.
There's a lot more info on this subject over at Musings of a Pertinacious Papist. Fascinating!