Charles Ellicott Commentary Luke 1:8

"Now it came to pass, while he executed the priest`s office before God in the order of his course," — Luke 1:8 (ASV)

In the order of his course.—This was settled by rotation. Attempts have been made by reckoning back from the date of the destruction of the Temple, when it is known that the "course" of Joiarib was ministering on the ninth day of the Jewish month Ab, to fix the precise date of the events narrated here, and therefore of our Lord’s Nativity, but all such attempts are necessarily more or less precarious.