According to the custom of the priest's office, his lot was to burn incense when he went into the temple of the Lord.
All Commentaries on Luke 1:9 Go To Luke 1
George Leo Haydock
AD 1849
It was his lot. The priests drew lots for the different functions to be performed in the same week; and now it fell by lot to Zacharias, to burn or offer up incense, morning and evening, in that part of the temple called the holy, where was the altar of incense: Zacharias was in this part of the tabernacle. (Witham)
See Exodus xxx. 6, 8.