Having completed their consecration period, Aaron and his sons began serving as Israel's priests.
We have previously established (here and here) the significant preparations that were necessary for Aaron and his sons to serve as Israel's priests.
Such measures were necessary because GOD is holy and does not allow sinful humans to march into His presence without our sins being covered.
For the nation, however, these preparations were only the beginning. Now that the priests were consecrated, the people had to prepare to approach GOD. Aaron and his sons led these rituals.
These events are recorded for us in Leviticus 9:
1 Now it came to pass on the eighth day that Moses called Aaron and his sons and Israel's council of elders. 2 Moses said to Aaron, “Take for yourself a young calf from the oxen as a sin offering and a ram as a whole burnt offering, without blemish, and offer them before the Lord. 3 And to the council of elders you shall speak, saying, ‘Take one kid of the goats as a sin offering, and a young calf and a lamb, both of the first year, without blemish, as a whole burnt offering, 4 also a young bull and a ram as peace offerings before the Lord, and a grain offering mixed with oil; for today the Lord will appear to you.’”
5 So they brought what Moses commanded before the tabernacle of testimony, and all the congregation drew near and stood before the Lord. 6 Then Moses spoke this word, “This is the thing the Lord commanded you to do, and the glory of the Lord will appear among you.” 7 Moses then said to Aaron, “Go to the altar, offer your sin offering and your whole burnt offering, and make atonement for yourself and for your house. Also offer the gifts of the people and make atonement for them, as the Lord commanded Moses.”
8 Aaron therefore went to the altar and killed the young calf of the sin offering. 9 Then Aaron's sons brought the blood to him; and he dipped his finger in the blood, put it on the horns of the altar, and poured the blood at the base of the altar. 10 But the fat, the kidneys, and the fatty lobe from the liver of the sin offering he offered on the altar, as the Lord commanded Moses. 11 The flesh and the hide he burned with fire outside the camp. 12 Then he killed the burnt offering; and Aaron's sons presented to him the blood, which he poured all around on the altar. 13 Then they presented the whole burnt offering to him, with its pieces and head, and he put them on the altar. 14 He washed the stomach and the feet and put them on the altar for a whole burnt offering.
15 Then he brought the people's gift and took the young goat, which was the sin offering for the people, and killed it and offered it for sin, like the first one. 16 He also brought the whole burnt offering and offered it according to the prescribed manner. 17 Then he brought the grain offering, took a handful of it, and put it on the altar, besides the whole burnt offering of the morning. 18 He also killed the young bull and the ram as sacrifices of peace offerings, which were for the people; and Aaron's sons presented to him the blood, which he poured all around on the altar. 19 The fat from the young bull and the ram: the fatty tail, what covers the stomach and the kidneys, and the fatty lobe attached to the liver, 20 they put on the breasts. Then he offered the fat on the altar; 21 but the breasts and the right thigh Aaron removed as a choice portion before the Lord, as the Lord commanded Moses.
22 Then Aaron lifted his hand toward the people, blessed them, and came down from offering the sin offering, the whole burnt offerings, and peace offerings. 23 After this, Moses and Aaron went into the tabernacle of testimony, and came out, and blessed all the people. Then the glory of the Lord appeared to all the people; 24 and fire came out from the Lord, and consumed the things on the altar, both the whole burnt offerings and the fat. When all the people saw this, they were amazed and fell on their faces.
(Leviticus 9:1-24 OSB)
GOD demonstrated His approval of the priests and His acceptance of Israel's collective sacrifice through the miraculous consumption of the offerings by fire. This sealed the role of the Levitical priesthood in Israel's government, moving forward.