It's the eighth day of the consecration of the Mishkan. There have been seven days of practice runs, with Moshe setting it up and performing the service (See VaYikra Ch. 8). However, this eighth day is to be its official opening, with Aharon running the proceedings. Moshe tells the expectant people who have all gathered: "The glory of the Lord will appear to you" (VaYikra 9:6).
However, Aharon seems a little hesitant. "Moses said to Aaron, "Approach the altar…So Aaron approached the altar" (ibid 7-8).
Rashi explains that "Aharon was ashamed and was scared to go forward" (Rashi on ibid) so Moshe needed to reassure and encourage him. The Ramban says that when Aharon saw the altar he actually saw the image of a bull (Ramban on ibid).
What is Aharon so unsure about?
In order to answer this question, we must first appreciate the day's tension. Israel has been building the Mishkan for six months. The whole purpose of the Miskan was so that God will "dwell amongst them" (Shemot 25:8). This is the day on which they will discover whether their hard work and sacrifice will come to fruition. Will God's presence actually rest on the Mishkan, or will it be an empty shell, signifying that God is not with them.
The man they all look towards is the high priest, Aharon. Yet Aharon has good reason to doubt himself. The previous time he acted as the people's mediator to God ended in disaster as he built the Golden Calf (ibid 32:5).
And now everywhere he goes, he is having flashbacks. Moshe tells Aharon that as his first act as high priest on that eight day is to "take for yourself a bull calf as a sin offering" (VaYikra 9:2).
While sin offerings are normally female sheep or goats, in this instance, Aharon must take a young calf. Rashi explains that is to show that he has been forgiven for the golden calf. However, for Aharon it is a reminder of his sin and rather than seeing that the positive, he sees the altar as an image of a calf. He asks himself whether he really is forgiven. Perhaps the gold on his clothing is another reminder.
However, Moshe reassures him: "Approach the altar and perform your sin offering and your burnt offering, atoning for yourself and for the people, and perform the people's sacrifice, atoning for them, as the Lord has commanded" (ibid 7).
Note how Moshe says: "atoning for yourself and for the people" and then just "perform the people's sacrifice, atoning for them". Moshe tells Aharon to bring the sin offering for you and the people, for the sin of the Golden Calf, but in reality, it is only the people that need atoning, his actions needed no atoning.
Indeed Aharon was vindicated: "fire went forth from before the Lord and consumed the burnt offering and the fats upon the altar, and all the people saw, sang praises, and fell upon their faces" (ibid 24).
Last year's Sedra Short on Parshat Shemini, entitled: "Aharon's Four Sons" appears at http://parshablog.blogspot.com/2007_04_01_archive.html
Another Sedra Short on Parshat Shemini, entitled: "How They Died" appears at http://parshablog.blogspot.com/2006_04_01_archive.html