We've presented this idea in the past, but we have a new angle from which to ponder it.
When did G-d tell Moshe that the Torah forbids a Jew from eating mixtures of meat cooked with milk? He told him during the 40 days and 40 nights that Moshe was on Har Sinai, from the day after Shavuot until the 17th of Tammuz.
When did Moshe tell B'nei Yisrael about Basar b'Chalav? Don't know exactly, but it was sometime after he came down from Har Sinai with the second Luchot and the Divine message of forgiveness for the golden calf. When did G-d tell us that Basar b'Chalav is forbidden? He told us in Parshat Mishpatim, Parshat Ki Tisa, and in Parshat R'ei.
We could ask the same questions about all the mitzvot, but we will let that one example start us off and lead us to the point.
Actually, let's take one more example. When did G-d tell Moshe about the prohibition of Gid HaNasheh? On Har Sinai during those 40 days and 40 nights. When did Moshe teach that prohibition to B'nei Yisrael of that first generation - a.k.a. Dor HaMidbar? The answer is some time during their time in the Wilderness. And when did G-d teach us that prohibition? The answer is in Parshat Vayishlach in the context of the story of the battle between Yaakov and the ISH (perhaps the guardian angel of Eisav).
This first point is that the first generation of Jews, the ones who physically stood at Har Sinai, the ones to whom Moshe taught the Torah. learned Torah and Mitzvot in a very different way from the way all generations since did, and from the way our generation does.
So when did G-d teach Moshe about the Mishkan and all of its details? According to one school of thought, He taught him on Har Sinai. But the people of that generation had no clue about making a Mikdash for HaShem until later. And when did they hear the commands and the details? On or after the first Yom Kippur in the Midbar.
AFTER the sin of the golden calf. And that is very significant for that generation. But what about us? When did G-d tell us and teach us and command us regarding making a Sanctuary for Him, so that He might dwell among us? He told all of that to us TWICE. Once, in Parshat T'ruma and Parshat T'tzaveh, and the first third of Parshat Ki Tisa. Then He told us and taught us about the Mikdash again, in Parshat Vayakhel and Parshat P'kudei. And we know very well what happened between the two times He taught us about Mikdash.
The Eigel HaZahav. And when did that happen? On the 17th of Tammuz 2448? Yes and no. It happened on that date. And that generation experienced it then, but we are told about it in Ki Tisa - every year. The question as to why we have all the details of Mikdash twice is a question for us, not for Dor HaMidbar. B'kitzur - we need both perspectives to hope to understand and appreciate the Mikdash. We have a different perspective from that of the original generation. We can relate to Mikdash - and we should - as both a Tikun and as an ideal.