THE TRUE BIRTHDAY OF CHRIST: IN OUR SEARCH FOR THE DATE OF CHRIST’S BIRTH, LET US TURN TO THE GOSPEL OF LUKE. Here we find the high priest named Zacharias performing his duties as priest and Luke gives us some details of when this was happening. “There was in the days of Herod, the king of Judaea, a certain priest named Zacharias, of the course of Abia: and his wife was of the daughters of Aaron, and her name was Elisabeth––-
And it came to pass, that while he executed the priest’s office before God in the order of his course, According to the custom of the priest’s office, his lot was to burn incense when he went into the temple of the Lord.” 
The time of this happening is given to us in the phrase “in the course of Ab-i-ah.” Now we need to go back in time to the days of David, who divided the performing of the priestly duties into twenty-four hour shifts, relays, or as stated in verse 5 and 8, “courses.” David, in 1 Chronicles 24, allocated sixteen of the courses the descendants of Eleazar and eight courses to the descendants of Ith-a-mar.
Each course was determined by casting lots and the first course fell to Jehoiarib (Je-hoy-a-rib) and “—the eighth to Abijah (a-bi-jah),” .
Each course administered for one week at a time and changes were made on the Sabbath day before the evening sacrifice , and all the courses were active during the main festivals. This reckoning or cycle began on the 22nd of Tis-ri. Reckoning from the 22nd of Tis-ri, the ministering in the temple would fall upon the course of A-bi-jah twice a year, from 6 to 12 December, and from 13-19 June in the year 5 B.C.