Jamieson, Fausset and Brown on Luke 1:9

9. his lot was to burn incense--The part assigned to each priest in his week of service was decided by lot. Three were employed at the offering of incense--to remove the ashes of the former service; to bring in and place on the golden altar the pan filled with hot burning coals taken from the altar of burnt offering; and to sprinkle the incense on the hot coals; and, while the smoke of it ascended, to make intercession for the people. This was the most distinguished part of the service (Re 8:3), and this was what fell to the lot of Zacharias at this time [Lightfoot].