In the year 5751, Tisha B’Av fell out on Shabbos (as it does this year, 5782). In such a year, the fast, together with all other aspects of mourning the destruction of the Beis Hamikdash, are pushed off until the 10th (Sunday). But the positive aspects of Tisha B’Av remain, which include it being an “auspicious time” (like every fast day) and also the birth of Moshiach. Therefore, on Shabbos Tisha B’Av we are eating a feast! Halacha permits one to sit and enjoy a repast as rich as Shlomo Hamelech’s even to the last moment before shkia (sunset) [unlike regular years, when we eat a mourner’s meal before the fast].
This itself is a taste of the Messianic Age, when all the fast days will be transformed to days of rejoicing, and as such our Shabbos meals on Tisha B’Av assume a Moshiach-like dimension of the feast of the Levyoson and Shor Habar (the Leviathan and Wild Bull). On a deeper level: every year we re-experience the churban, the destruction of the Beis Hamikdash, and the mourning, and then in the afternoon, at Mincha-time, comes the comforting — the birth of Moshiach. This year we have the birth of Moshiach (who builds the 3rd Beis Hamikdash which will never be destroyed) without the churban! This hints that b’pnimiyus the destruction of the 1st and 2nd Beis Hamikdash were really only for the sake of the building of the 3rd, eternal Beis Hamikdash.
All of this was in fact present at the time of the churban, but it was concealed. The inner positive aspects of Tisha B’Av, the birth of Moshiach, does not become revealed until the 15th of Av, which is when the moon is full, the moon being the aspect of Dovid Malka Mashicha. (This also hints at the Jewish people, when they reach full potential to receive the light (as the moon receives and reflects from the sun), reveal that Moshiach was actually born on Tisha B’Av.
The three weeks leading up to Tisha B’Av hint at the “three moichin” (3 aspects of Supernal Intellect) which themselves correspond to the lands of the three nations — Keni, Kenizi, Kadmoni — which were promised to Avraham Avinu (along with the land of 7 nations conquered by Yehoshua). The Torah forbids us to have anything to do with these lands during the time of until Moshiach comes, but the fact that part of these lands were conquered by other nations (Sichon and Og) and then conquered and settled by the tribes of Gad, Reuven, and Menashe, is itself a hint that these three dimensions of the Supernal Intellect come together with the land of 7 nations, which refers to the seven midos.
Sefer Devarim is Moshe Rabbeinu addressing the Jewish nation before they will enter the land of Israel. Moshe knew that Hashem had decreed upon him not to enter the land, but still he did not cease to beseech Hashem in prayer asking Him to allow him to enter the land, reaching 515 prayers (the numerical value of “Va’eschanon”, the parsha we read at Mincha).
From here there is also something for all generations to learn from… that without looking at all the prayers and requests that were until now, we need again and again to daven and request from the Eybershter “Ad Mosai” — “until when”.
…and as was said, immediately mamash, today mamash, since all the matters have been completed, and the Beis Hamikdash stands ready Above, and similarly all the matters — “everything is ready for the feast”, everything is ready as if in a closed chest and they have given the chest and its key to every single Jew. The only thing that we are waiting for — that a Jew will shout another shout, with another request and demand and another reminder: “Ad Mosai“?!…
How long are we prepared to wait? Ad mosai?!