“According to the custom of the priest’s office, his lot was to burn incense when he went into the temple of the Lord.”
Luke 1:9 — Greek Interlinear
Church Fathers on Luke 1:9
The Lord appointed by the hand of Moses one High Priest, at whose death another was to succeed in due order. This was observed until the time of David, who by the command of the Lord increased the number of the Priests; and so at this time Zacharias is said to have been performing his Priest’s office in the order of his course, as it follows: But it came to pass, when Zacharias was performing the Priest’s office in the order of his course before God, according to the custom…
Zacharias seems here to be designated High Priest, because into the second tabernacle went the High Priest alone once every year, not without blood, which he offered for himself and the sins of the people.