The Seer of Lublin passed away, at the age of 70, on the 9th of Av 5575 (1815), a day of national mourning, but also, according to the sages, the birthday of the Mashiach. Long before his passing he hinted to his followers that he would pass away on the 9th of Av.
The Seer of Lublin, the mentor of almost all the leaders of the major Chasidic dynasties in Poland in the beginning of the 19th century (according to tradition, 120 Chasidic Rebbes were his disciples), was one of those great tzadikim who, like the Lubavitcher Rebbe in our generation, out of infinite love and compassion for the Jewish people, devoted his life, with total self-sacrifice (to the extent of becoming physically ill…), to bringing the Mashiach – now!
In Kabbalah, the passing of a tzadik is understood as the elevation of "feminine waters" (the existential longing of the tzadik's soul to leave the confines of the physical body, to return to and become part of God) that arouses from on high the descent of "masculine waters" (God's arousal, as it were, to impregnate reality with a "new soul" of a great tzadik who will shine "new light" to the world).
And so, it is most appropriate that on the very day of the passing of the Seer of Lublin (the saddest day of the year) the soul of the Mashiach (the happiest of all souls; the letters of Mashiach [משיח] permute to spell "he will be happy" [יִשְמַח] and "he will make others happy" [יְשַמַח]) is born, or perhaps better put, conceived on the spiritual plane.
According to the sages, from conception to "the formation of the fetus" (when the major features of the fetus become well defined in the womb) takes 40 days. 40 days beginning from the 9th of Av brings us to the 18th of Elul.
On the 18th of Elul 5369 (1609), the Maharal of Prague passed away. On that very same day, 89 years later (the 18th of Elul 5458), the Ba'al Shem Tov was born. 47 years later, on the same day (the 18th of Elul 5505), Rabbi Shneur Zalman of Liadi, the first Lubavitcher Rebbe, was born.
Similarly, on the 7th of Adar Moses passed away, on his own birthday, at the age of 120. We are taught that not only was he born 120 years before his passing, but that in fact every year he is born anew on that very same day, the 7th of Adar.
So we see that the passing of a tzadik and the birth (or conception) of a new tzadik go together. The new tzadik mirrors the essence of the first tzadik's deepest devotion in life. And so, if the first tzadik sacrificed his life to bring Mashiach, as did the Seer of Lublin, then indeed the Mashiach is born (conceived) on the day of his ascent to heaven (as in the conception that takes place in marital union).
The passing of the Seer of Lublin joins together with the "passing" of the Divine Presence from the Holy Temple in Jerusalem on the 9th of Av (the day of the destruction of the Temple – only its physical body "died" but its soul ascended to heaven) to arouse God to bring the Mashiach (who will permeate reality with Divine revelation, bringing redemption, peace and goodness to all) – now!