(Lev. 9:1) "And it came to pass on the eighth day that Moses called etc." Our Rabbis said: During all those days that Moses was at the thornbush, the Holy One, blessed be He, said to him, "Go on My errand," and he kept saying, "Please send by the hand of him whom You will send" (Exodus 4:13). So too on the first day and the second day. The Holy One, blessed be He, said to him, "I am coaxing you, and you say, 'Send.' By your life, I will repay you for it: tomorrow, when the Tabernacle is made, you will think in yourself that you will serve in the high priesthood, but I will say to you, 'Call Aaron, that he may serve.'" Therefore it is said, "Moses called Aaron and his sons."
There is a calling that is for plenty, [and there is a calling for famine. Whence for plenty?] As it is said, "And I will call to the grain and multiply it" (Ezekiel 36:29). Whence for famine? [As it is said,] "The LORD has called for a famine etc." (II Kings 8:1). There is a calling for greatness, as it is said, "Moses called." Moses said to him, "Thus the Holy One, blessed be He, said to me: to appoint you as High Priest." Aaron said to him, "You labored at the Tabernacle, and I am to become High Priest?" He said to him, "By your life, even though you become High Priest, it is as though I become it; for just as you rejoiced for me in my greatness, so I rejoice for you in your greatness."
And when did he rejoice for him? At the hour when the Holy One, blessed be He, said to him, "And now come, and I will send you to Pharaoh" (Exodus 3:10). The Holy One, blessed be He, said to him, "Go; the matter is held in reserve." Moses said, "Please, my Lord" (Exodus 4:10) — "bayya, you are passing this over upon me: my brother is greater than I, and I go before him." The Holy One, blessed be He, said to him, "[By your life,] you have spoken fittingly, and even so, 'he will see you and rejoice in his heart'" (Exodus 4:14). Rabbi Shimon ben Yohai said: The Holy One, blessed be He, said to him, "That heart which rejoiced at the greatness of his brother — they shall set those precious stones upon it." Therefore, all those seven days that Moses was occupied with the Tabernacle, he was throwing the blood and burning the fats. The Holy One, blessed be He, said to him, "What do you suppose, that you will be High Priest? Call your brother, that he may serve as High Priest." Therefore, "And it came to pass on the eighth day that Moses called [Aaron and his sons and the elders of Israel]." And why "to the elders of Israel"? In order to exalt him before the elders. The Holy One, blessed be He, said to him, "Call the elders and appoint him before them, so that Israel will not say, 'He became High Priest of his own accord.'" Therefore, "and the elders of Israel."