The Talmud states (Taanit 26b) "When Av begins, we reduce in joy.".
The simplest explanation of this phrase is that as soon as the month of Av begins, we should express a certain degree of mourning for the events that occurred throughout Jewish history on the 9th of Av.
Furthermore, the Talmud explains that this is not a good time to start new businesses or ventures in general, as the Jewish people's "mazal" is weak. The word "mazal" is generally translated as luck, but the basic idea in this case is that the energy of that day is such that it attracts the most difficult events. It's like old pinball machines: if there was a magnet under the board, the metal balls would always be drawn to that spot, making the game more challenging. In the same way, the 9th of Av is a magnet for negative events, and its influence begins to be felt at the beginning of the month of Av. That is why, "When Av begins, we reduce our joy.".
However, there is a Hasidic explanation of this phrase from the Talmud that, without taking away its simple meaning and without taking away the solemnity that halakha - Jewish law - applies to this period, offers us a very profound vision of the perspective of Hasidic life.
Hasidism says that the Talmudic phrase can be read "When Av begins, we reduce, with joy" (note the change in the comma's position and the change from "in joy" to "with joy").
In Hebrew the Talmud says "beSimjá", which can be translated both "in joy" (which is the simple meaning) and "with joy" (deeper meaning).
The idea is that at the beginning of the month of Av we should reduce certain aspects of our daily behavior, such as not eating meat and drinking wine, bathing, etc. However, explains Hasidic wisdom, this reduction should be done "with joy".
Now, if this is a time of sadness, how can it be lessened "with joy"?
King David wrote in his Psalms, "The precepts of the Lord are right, rejoicing the heart." We learn from this that there is an inherent joy in studying and observing God's commandments. This is, so to speak, a "kosher" joy for this period between the 1st and 9th of Av.
That is the meaning of the Talmudic phrase, from the Hasidic perspective: we must reduce our pleasurable activities, but along with the joy of Torah and, even more so, with a constant increase of this "kosher" and appropriate joy, until these days are transformed into days of rejoicing and gladness for all the Jewish people, with the coming of our righteous redeemer, speedily in our days.