Sources where the Rebbe proclaims that Moshiach has already been revealed. * Chapter Seven of Rabbi Shloma Majeski ’s Likkutei Mekoros (Underlined text is the compiler’s emphasis.)
Translated by Boruch Merkur
8. […] In addition to the fact that we are presently right before the true and complete redemption, this year is “(T’hei) Shnas Niflaos Er’enu – (It shall be) the year of I shall show you wonders” (the acronym for the year 5751 that has been globally accepted by the Jewish people). This is the year in which the promise of “as in the days of your exodus from Egypt, I shall show you wonders” will be fulfilled. In fact, we have plainly seen “wonders” testifying that this is the “year in which Melech HaMoshiach is revealed.” Indeed, “at the time of the advent of Melech HaMoshiach … he proclaims to them, to the Jewish people, ‘Humble ones, the time of your redemption has arrived” (as in the words of the Yalkut Shimoni), leading up to the proclamation that “Behold, he (Melech HaMoshiach) has come,” that he has already arrived. That is, we are presently on the very threshold of the beginning of the Messianic Era, the threshold of the redemption, and immediately thereafter, its continuation and its perfection.
Since this is so, certainly Yud-Beis Tammuz (which occurs on Shabbos this year), as well as the entire Three Weeks of Bein HaMeitzarim of this year, emphasize the concept of redemption. And not just the preparation for the redemption but the actual advent of the redemption takes place on this Shabbos Kodesh, even prior to the beginning of (Bein Ha)Meitzarim. These days will, therefore, be transformed into days of celebration and rejoicing, and festive holidays.
(From the address of Shabbos Parshas Balak, 17 (Tov) Tammuz; Seifer HaSichos 5751, pg. 690)