Parashat Shemini discusses the dedication of the mishkan, and opens with this verse:
“On the eighth day Moses called Aaron and his sons, and the elders of Israel.” (Vayikra 9:1)
This leads to an obvious question – on the eighth day of what?
The answer is presumably found a few verses earlier, at the end of Parashat Tzav. Moshe tells Aharon and his sons:
“You shall not go outside the entrance of the Tent of Meeting for seven days, until the day that your period of ordination is completed. For your ordination will require seven days. Everything done today, the LORD has commanded to be done [seven days], to make expiation for you. You shall remain at the entrance of the Tent of Meeting day and night for seven days, keeping the LORD’s charge—that you may not die—for so I have been commanded.” (Vayikra 8:33-35)
After the priests’ seven days of ordination (“miluim”), on the eighth day the mishkan was dedicated.
However, the questions don’t end there. We read a parallel version of this narrative at the end of the book of Shemot. God tells Moshe to set up the mishkan on the first day of the first month (1 Nissan in our calendar):
“On the first day of the first month you shall set up the Tabernacle of the Tent of Meeting.” (Shemot 40:2)
And Moshe does just that later in the chapter:
“In the first month of the second year, on the first of the month, the Tabernacle was set up.” (Shemot 40:17)
And apparently on the same day, God’s presence filled the mishkan:
“The cloud covered the Tent of Meeting, and the Presence of the LORD filled the Tabernacle.” (Shemot 40:34)
If the mishkan was set up on 1 Nissan, when was the eighth day mentioned at the beginning of Parashat Shemini?
There are two opinions offered by the Talmudic sages, and the medieval commentators continued that debate.
One opinion (by Rabbi Akiva in the Sifrei, and expanded up on by Ibn Ezra in his commentary to Vayikra 9:1 and his long commentary to Shemot 40:2) says that the seven days of milium must have come after the mishkan was set up, so the eighth day must have been 8 Nissan.
But Rashi (following the other sages in the midrash) says that the eighth day was the same day that the mishkan was set up, 1 Nissan:
“And it came to pass on the eight day of the installation of the priests into their sacred office (cf. Sifra); this was the New Moon of Nisan on which the Tabernacle was finally erected and it (that day) received ten crowns.” (Rashi on Vayikra 9:10)
So according to this opinion, the first day of the miluim was 7 days earlier – 23 Adar.
As Ibn Ezra (and other commentaries) point out, this does not seem to be the plain meaning of the text. There are a number of reasons, but primarily it is difficult to reconcile the mishkan being set up on the same day as the dedication. What was happening during the seven days before that? The answer offered (Sifrei Bamidbar 7:1) is that for seven days, the mishkan was set up and then dismantled, and on the eighth day it was set up permanently.
So why did those rabbis insist on saying the eighth day was 1 Nissan instead of the more plausible 8 Nissan?
I think the answer can be found at the end of Rashi’s comment, regarding the “ten crowns.” He took this idea from a Talmudic passage:
“That day took ten crowns. It was the first day of Creation, meaning Sunday, the first day of the offerings brought by the princes, the first day of the priesthood, the first day of service in the Temple, the first time for the descent of fire onto the altar, the first time that consecrated foods were eaten, the first day of the resting of the Divine Presence upon the Jewish people, the first day that the Jewish people were blessed by the priests, and the first day of the prohibition to bring offerings on improvised altars.” (Shabbat 87b)
This passage is teaching us that the 1st of Nissan of that year not only inaugurated the mishkan, but was the first in ten different ways. The service in the mishkan was not the continuation of previous historical events, but rather began an entirely new era – it was sui generis.
This may also explain why the Sages chose to begin Parashat Shemini where they did. Instead of starting with the first day of the milium, a new parsha began on the eighth day, at the dedication of the mishkan.
The question of how to look at the ritual practices in the mishkan was the source of significant controversy among Jewish thinkers. Rambam famously claimed that the sacrifices were instituted in response to the idolatrous practices at the time. The Torah allowed Israel to continue to sacrifice, but in a way that served God instead of foreign gods. Others objected strongly to this approach, saying that the mitzvot of the mishkan were given for their own higher purposes, not as a rebuttal to the actions of others. That perception of the mishkan is reflected in how they understand the eighth day as well.