On the Biblical calendar, the fifth month of the year (counting from Nisan) is called Av (אָב) in Jewish tradition. The month of Av is traditionally regarded as the most tragic in the Jewish calendar. On the first day of this month, Aaron (the first High Priest of Israel) died (Num. 33:38), and later both of the Holy Temples were destroyed on the ninth of Av.
The ninth of Av (i.e., Tishah B'Av) is the lowest point of a three week period of mourning that began with the fast of the 17th of Tammuz (undertaken to recall the first breach in the walls of Jerusalem by the Babylonians before the First Temple was destroyed). During this period, weddings and parties are forbidden. It is a time for solemn reflection and mourning for Israel. The Temple itself was destroyed three weeks later, on the Ninth of Av.
The Three Weeks of Sorrow
As mentioned above, the Fast of Tammuz marks the beginning of the "Three Weeks of Sorrow," a 21 day period of national mourning which is completed on Tishah B'Av. The Mishnah further associates the fast of Tammuz as the "Fast of the Fourth Month," mentioned by the prophet Zechariah (Zech. 8:19). The purpose of this fast is to instill a sense of teshuvah (repentance) by recalling specific tragedies that befell the Jewish people because of idolatry.
The period between the fourth and fifth months is a somber time of reflection for many Orthodox Jews. The three weeks from Tammuz 17 to the Av 9 is called bein ha-Metzarim (בֵּין הַמְּצָרִים) - "between the straights" (based on Lamentations 1:3), a period of time during which many calamities befell the Jewish people. Since both Temples were destroyed during this period (i.e., between the 4th and 5th months), the sages established this extended period as a time of mourning for the Jewish people.
Typically marriages are not held during this period, and many Jews deliberately refrain from ostensibly pleasurable activities, such as listening to music, dancing, taking vacations, and sometimes even shaving! In fact, most Orthodox Jews will refrain from any activity that might require the recitation of the Shehecheyanu blessing.
In short, the Three Weeks of Sorrow is a time for reflection and mourning over the destruction of the Temple and therefore constitutes a time of corporate reflection intended to lead Israel to teshuvah (repentance).
The period immediately following Tishah B'Av and continuing through the month of Elul is called the "seven weeks of comfort" (shiva d'nechemta: שְׁבְעָה דנחמתא). The Haftarah for each week during this period foretells of the glorious future of the Jewish People. Because of this, some Jews use the name "Av" through the Tishah B'Av and afterwards use the name "Menachem Av" (i.e., the Comforter of Av).
In synagogue services the Haftarot (readings from the Nevi'im, or prophets) are usually connected with the Torah reading for each week, but beginning with the Three Weeks of Sorrow until after the Fall High Holidays, these readings change. First are three haftarot of punishment (leading up to Tishah B'Av), then seven of comfort (for the seven weeks until Rosh Hashanah), and finally one of repentance (on Shabbat Shuvah, the Sabbath before Yom Kippur).
The last two portions of the Book of Numbers (Mattot and Masei) are always read during the "Three Weeks of Sorrow." The sages say these readings were selected at this time to ultimately comfort us as we look forward to the "apportioning of the land" -- i.e., the ultimate fulfillment of God's promises to us. Indeed, the month of Av -- despite the sorrow of the loss of the Temple -- is sometimes called Menachem Av - the "Comfort of the Father" (אָב). One day the lamentations of our present state of exile will come to an end.
Menachem Av may also mean the "comfort of Aleph-Bet" (אב). The Prophet Jeremiah, who witnessed the destruction of the Temple, later wrote the scroll of Lamentations to commemorate this tragic time. The form of Lamentations is an acrostic based on the letters of the Hebrew Aleph Bet (like Psalm 25, 34, 37, 119, Prov. 31, etc.). The scroll has five sections (perekim). The verses of the first two chapters and the last two chapters all are written in alphabetical order (א,ב,ג). The middle chapter, however, writes its verses using a triple Aleph Bet ordering, i.e., "Aleph, Aleph, Aleph," "Bet, Bet, Bet," "Gimel, Gimel, Gimel," and so on. As you can see, the very order of the Hebrew Alphabet is part of the revelation of the LORD God of Israel Himself!
The low point of the Three Weeks of Sorrow is expressed during the 25-hour fast day of Tishah B'Av, when the Scroll of Lamentations (אֵיכָה) is chanted during the evening service at synagogue. This is usually followed by a series of liturgical lamentations called Kinnot (קִינוֹת) which are also recited alphabetically (Bava Batra 14b). As I've said before, Tishah B'Av is the saddest day of the Jewish calendar.
But it is not a sadness without a comfort... The Sabbath immediately following Tishah B'Av is called שבת נחמו (Shabbat Nachamu - the "Sabbath of Comfort"), based on the Haftarah reading (Isaiah 40:1-27) that begins: נַחֲמוּ נַחֲמוּ עַמִּי - Nachamu, Nachamu ami - "be comforted, be comforted, my people..." The sages state that the word nachamu is repeated to offer consolation for each of the Temples that were destroyed.
Since the Book of Deuteronomy is mishneh Torah - a "retelling of Torah," it can be said that the Torah -- from a narrative point of view -- ends with the reading of these final portions from Numbers, and by extension, with the yearning for Zion. And so it is to this day. We await the return of our Mashiach Yeshua while we live in exile here on earth. And even though the Temple of the LORD is spiritually present in the Person of the resurrected Messiah, it will be made fully manifest in the days to come: first in the Millennial Kingdom (after Yeshua's Second Coming), and later still in olam habah (the world to come) as the eternal community of those redeemed by the Lamb of God (Rev. 21:22-23). So for those of us who hold faith in Yeshua as Messiah, our mourning for the Temple is really mourning for the Presence of our Beloved Savior.
May He come quickly, and in our days...