The 15th of Av is undoubtedly the most mysterious day of the
Jewish calendar. A search of the Shulchan Aruch (“Code of Jewish Law”) reveals no observances or customs for this date, except for the instruction that the tachanun
(confession of sins) and similar portions should be omitted from the
daily prayers (as is the case with all festive dates), and that
beginning on the 15th of Av one should increase one’s study of Torah, since at this time of the year the nights begin to grow longer, and “the night was created for study.” And the Talmud tells us that many years ago the “daughters of Jerusalem would go dance in the vineyards” on the 15th of Av, and “whoever
did not have a wife would go there” to find himself a bride.
And this is the day which the Talmud considers the greatest
festival of the year, with Yom Kippur (!) a close second!
Indeed, the 15th of Av cannot but be a mystery. As the “full
moon” of the tragic month of Av, it is the festival of the Future
Redemption, and thus a day whose essence, by definition, is unknowable
to our unredeemed selves.
Yet also the unknowable is ours to seek and explore, as we shall in the essays, based on the Lubavitcher Rebbe’s writings and talks, presented here.
source: chabad.org