The latter half of Parshat Tzav deals with the 7 days of preparing Aharon and his sons for the kehuna, with the 8th day being in Parshat Shmini, where the transfer is actually made.
In Shir Ha'Shirim in 3:11, the passuk says, "צאינה וראינה בנות ציון..וביום שמחת לבו"- go out and gaze.. upon the crown with which his mother crowned him on the day of his wedding and on the day of the happiness (joy) of his heart.
Rashi tells us "the day of his wedding" refers to the day of the giving of the Torah, and "the day of the happiness of his heart" refers to the 8th day of the consecration, on which the mishkan was dedicated.
The 8th day, the day the transfer of the kehuna was completed and the day the mishkan was established was an especially joyous day. Shlomo Ha'Melech calls it the day of joy of his heart.
The 7 days leading up to the 8th, which are described in parshat Tzav, must have been nerve-racking, along with being frightening and inspiring at the same time. They are getting ready for the most monumentous day in history, second to none, other than Matan Torah.
Can you imagine the trepidation with which they must have performed their obligations? As Moshe is dressing Aharon and his sons in the priestly garments, making sure everything is just so, and as he is training him in the various korbanos and their laws and procedures, the thoughts that must have been going through their heads!
When people prepare for a production, in the days leading up to the performance, their level of nervousness and fear increase more each day, until it is diffused on stage at the actual performance. How much more so they must have been nervous when preparing for "opening day" at the mishkan, knowing that Bnei Yisrael, and their relationship with Hashem, would depend on Aharon's performance.
Yet, I believe, that is why it tells us, "כאשר צוה ה את משה" - as Hashem commanded Moshe - every step of the way. Moshe said, this is what Hashem told me to do. he got Aharon dressed, as Hashem commanded Moshe. He placed upon him the hoshen, as Hashem commanded Moshe. They brought certain korbanos, as Hashem commanded Moshe. And so on.
This was not a performance. They were not putting on a production, where they needed to fear mistakes, failure and embarrassment. They were following the instructions of Hashem. Exactly. To the smallest point. Therefore they did it with joy and excitement, and not with fear and nervousness.
And the climax was on the 8th day when the Mishkan was opened and the love between Hashem and the people was even tangible, manifested through the new temple. וביום שמחת לבו.