In the beginning of Parshas Terumah, the Torah commands all of Klal Yisrael to participate in the building of the Mishkan. The Rambam (Hilchos Beis Habechira 1:4) codifies this commandment, writing that every Jew must participate in the building of the Mikdash, either by physically building it or contributing monetarily.
A mishnah in Rosh Hashanah (30a) states that when the Beis Hamikdash stood, new grain (chadash) became permitted with the bringing of the korban Omer. After the destruction of the Beis Hamikdash, Rav Yochanon Ben Zakai decreed that new grain is only permitted after the second day of Pesach. The Gemara explains that we wait because there is a distinct possibility that the third Beis Hamikdash will be built on the first day of Pesach.
Rashi asks: If the halacha is that building the Beis Hamikdash is prohibited on Shabbos and Yom Tov, how can the Gemara contemplate that the third Beis Hamikdash will be built on Yom Tov? Rashi answers by quoting a Chazal that the third Beis Hamikdash will descend as fire from heaven. (Rashi’s source seems to be the Bava Kama 62a.)
The Aruch Laner asks: How can Rashi say that the third Beis Hamikdash will descend as fire from heaven when every Jew prays several times a day for the rebuilding of the Beis Hamikdash? He also points out that Rashi’s comment seems to contradict the Gemara (Sukka 52a), which describes Mashiach as a master architect who will spearhead the building of the third Beis Hamikdash. If the Beis Hamikdash will descend from heaven, why is it important that Mashiach be a master architect?
The Aruch Laner, therefore, offers a novel interpretation of Chazal’s statement regarding the Beis Hamikdash descending from heaven. He suggests that we will build the Beis Hamikdash; however, the kedushah of the Beis Hamikdash will descend as fire from heaven, thus electrifying the building its Beis Hamikdash status. (The first and second batei mikdash, in contrast, attained their kedushah as they were being built.)
This explanation seems difficult to understand, though, since it revives the question Rashi asked: How can the Gemara contemplate the third Beis Hamikdash being built on Yom Tov?
I heard the following answer from Rav Shlomo Cynamon: Tosafos (Shabbos 95b) maintains that building is actually permitted m’d’Oraisa on Yom Tov; it’s only prohibited m’drabanon. The only problem with building a complete building on Yom Tov – argues the Chasam Sofer – is maka b’patish (the final bang). When one puts the final touches on a building – i.e. affixing the doors or final windows – one transgresses the melachah of maka b’patish.
But according to the Aruch Laner, there is no problem of maka b’patish. Why not? Because the maka b’patish in the case of the Beis Hamikdash is the kedushah that descends upon it. This kedushah completes the building. It’s the “final bang.” But according to the Aruch Laner, this “final bang” is performed by heaven, not man. Thus, building the third Beis Hamikdash on Yom Tov is permitted.