The Fifteenth of Av marks a turning point -- it is the summer equinox. This means that until this day, the days are longer than the nights. On the Fifteenth of Av, the day and night are equal, and from this point on, the days become shorter and the nights longer. Now, what is the significance of that to a Yid?
Since "nights were given for learning Torah," in the words of our Sages, this indicates that from this point on we must increase our Torah learning.
It is interesting that people say, "I'm a day person," or "I'm a night person," or "I'm an early morning person." We definitely are aware of the fact that the time of day has an effect on the way we perform and behave. Generally, the daytime is when people interact with the world. You go out, shop, work, and do things. What do people generally do at night? Night is usually the time of rest and relaxation. Most people, except the minority who have night jobs, spend their time at home at night ideally with their families, but in any event, in a quieter way than they spend their day. Since the learning of Torah can be done best with concentration, and you're finished with your day's work, this is the proper and suitable time for the study of Torah. The fact that from Tu BeAv on, the night becomes longer than the day, is almost a subtle hint and reminder to a person that now there is more time to learn more than there was before Tu BeAv. Now is the time to make an increase in the learning of Torah.
The Rebbe adds that since Tu BeAv marks the beginning of the second half of Av it may be regarded as the "eve" of the month of Elul, the last month of the year, in which we prepare for the new year, which is Tishrei. Accordingly, preparations for Elul and Tishrei begin from this point on as well.
The Rebbe cites the Tanya (see ch. 5) that when a person is learning Torah, his brain is involved with HaShem's wisdom. Someone once told me that a person's brain capacity is enormous. Each of us has stored up in our brains endless bits of information. Only the thing you are learning now is in the forefront of your brain. What you learned 25 years ago has receded into the storage areas of the brain, but whatever aspect of HaShem's wisdom you are directly involved with now, is the vehicle through which you can unite yourself with HaShem at this minute.
The Rebbe mentioned this so that a person can understand the value of learning Torah. A finite human being who isn't brilliant may be of merely average intelligence, but if he nevertheless chooses to busy himself with Torah, he is at that moment in a yichud nifla, a fantastic union, with HaShem. As long as he can keep this up, he is on this high level.
In Yalkut Shimoni, HaShem refers to the Jewish people as anavim, humble people. Of all the different wonderful qualities which characterize the Jewish people, HaShem calls us anavim, rather than tzaddikim (righteous), chachamim (wise), etc. because humility, specifically, is the quality that is required for achieving greatness in Yiddishkeit. The Rebbe explains that if a Jew does not have humility, then he can never attain true wholeness and completeness. As we know, some people who learn Torah are quite arrogant. A person's arrogance may actually be the result of learning Torah -- people call him a ben Torah, a talmid chacham, etc., and he begins to feel very proud of how much Torah he knows.
The Rebbe adds that if one who is learning Torah suffers from pride when he learns Torah, then not only does the Torah he learns not elevate him, but he lowers the Torah to his level. In other words, the benefit of learning Torah depends on the learner. If he is a person who has humility, then the Torah raises him; if he is arrogant, he degrades the Torah to his level, which is a very low level since arrogance is one of the traits most despised in the Torah. When a person is arrogant, he is suffering from a terrible disease; in fact, the Rebbe compares arrogance to a malignant disease.
HaShem declares that He cannot dwell in the presence of arrogance, say our Sages. In Chassidus it is said that a person who is arrogant is so full of himself that he does not allow room for anybody else, including HaShem. So HaShem says, "If so, I'll go somewhere else." That is why the prerequisite for successful Torah learning is to have anivus, humility.
There is a story told about a gifted Torah scholar who came up with a question to which he could not find the answer. He went from scholar to scholar, from Rosh Yeshivah to Rosh Yeshivah over a long period of time, but still did not find a satisfactory answer. Eventually, although he was far from being a chassid (in fact he was a misnaged), someone convinced him to present his question to the Alter Rebbe. He traveled to Liadi, where he spent an unusually long period of time. When he eventually returned home, he was a different person altogether. His family and friends bombarded him with questions, "Nu! What took you so long there? What has happened to you, you seem to have become a different person altogether! What did the Rebbe answer you?"
"Before I went to the Rebbe, to Liadi," he said, "I had a question for which I had only an unsatisfactory answer. Now that I have spent some time with the Rebbe, I still have a question for which I have only an unsatisfactory answer. However, although the answer is the same answer, the question is a different question." He then explained his cryptic remarks: "Before I went to the Rebbe, my question was this -- it states in the holy books that for even a single mitzvah that a person does, and for even the little Torah that he learns, he is given infinite reward. Now, since I have learned so much Torah, and since I have so many mitzvos to my credit, I could not understand how HaShem would manage to reward me for all of this. My answer then, although unsatisfactory, was: 'Since HaShem is omnipotent, He will surely find a way to reward me.' Then I went to the Rebbe. Now my question is, 'How can HaShem stand to have someone like me in His universe?' And the answer is, 'Since HaShem is omnipotent, He has found a way to manage that too!' And that, incidentally, that is why I am now a Chabad chassid!"
It is well worth keeping these matters in mind as we increase our Torah learning from the Fifteenth of Av onwards!