I suggest distinguishing two types of mourning. The first type entails memories that pale as time passes. Do you recall the Massacre of the Jews of York in 1190? How about the destruction in 1391 of the Jewish communities of Spain? Probably not. Tragedies fade away. Time diminishes pain. Similarly, when a person mourns a relative, there is a gradual lessening of mourning from the immediate mourning of onan, to shivah (1st seven days) to shloshim (30 days) and finally to yahrtzeit (yearly observance). Memory dissipates. Time heals.
However, there is a second type of mourning. It is functional and situational. It implies remembering a glorious setting that has vanished, grieving a beautiful intimacy that has been destroyed. Such mourning serves a different purpose. It keeps hope alive. It stimulates action to reconstruct a new and better home. It enjoins reflecting upon mistakes and learning from past errors. It provides a framework for dedicated work to recreate the idealized past. This type of mourning is a major function of Tisha b'Av.
The prototype of Tisha b'Av is to remember our exile from Israel and the destruction of the Temple as a way of encouraging return, rebuilding and teshuva. Thus, today, when through our struggles we have returned to the land of our ancestors, rebuilt our sovereignty and can rejoice in Jerusalem, Tisha b'Av has acquired a totally new meaning.
At weddings, a time of great joy and expectation, we break a glass and recite the verse from Psalms 137, 5-6, "If I Forget Thee, O Jerusalem let my right hand be forgotten…". This is based on the Talmudic mandate (Babba Batra 60b) interpreting the verse at the end of Isaiah (66,10) to mean that great joy is promised to those who both love and mourn Jerusalem. By remembering the past we give appreciation to the present and anticipate the future. This is a bridge over and beyond time that gives meaning to our joys in the present and inspirational comfort towards the future.
I would like to address your question from my personal vantage point here in Jerusalem. I live on French Hill-Mt. Scopus, and every day, as I exercise in the sports center at the Hebrew U. gym, I can look out the window and see the Temple Mt. with the sweep of Jewish history, the mythological grandeur of the past and the utopian vision of the future. Breathing in the fresh air of Jerusalem, we are free to dance in the streets. Jeremiah 33, 10-11: In this place that you say is desolate, in the cities of Judea and in the outskirts of Jerusalem, the sounds of rejoicing and happiness will be heard … This is the prophecy of Zechariah 7,19 that the four Fast Days of mourning are destined to be transformed into a time of rejoicing, joy and holidays.
Here in Israel, the memory of the Temple's destruction has taken on a new national ethos. Reciting the lamentations now gives poignant meaning to the Messianic anticipation. It is out of the darkness that we can envision the dawn of redemption. This is mourning over the lost "situation", and the goal is to restore the joy and grandeur. The restoration of national independence, the freedom to walk in Jerusalem, the pride in the revival of Jewish history, are all small steps towards the fulfillment of a grand Messianic vision of national purpose and spiritual meaning.
Rabbi Shlomo Carlebach (1925-1994), the great singer and poetic story teller, described Tisha b'Av as a harbinger of utopian rejuvenation, a hope for renewed intimacy, an anticipation of Messiah:
Jerusalem, the Holy Temple, is where we are intimate with God. The Holy Temple is the headquarters for being close to God and to each other. But when the house is destroyed, there is no place to be intimate anymore. And gevalt! Are we longing and crying to be intimate with God …On Tisha b'Av the Messiah comes. On Tisha b’Av until the Six Million you only heard the sound of the destruction of the Temple; you could not hear the footsteps of the Messiah. Today, the voice of destruction gets further and further away, the voice of the coming of the Messiah gets closer and closer. The utopian message is that the whole world is being fixed as God’s holy intimacy comes back into the world and into our lives (Aug. 3, 1992, Congregation Kehillat Jacob News).
In this reinterpretation, Tisha b'Av is the harbinger of the utopian message that intimacy with God can be regained and with it the fixing of our lives and of the whole world. This then, is a "mourning" that is filled with hopeful joy.
Thus, to answer your question, this type of Tisha b'Av is a "mourning" that is a harbinger of hopeful joy.
Answered by: Rabbi Dr. Natan Ophir