The Nature and Uniqueness of the Three Weeks:
The Three Weeks are days of atonement. Where can one learn about this? The Three Weeks lasts approximately 21 days, from the 17th of Tammuz until Tisha Be'Av. There are another 21 days that we know - the 21 days from Rosh Hashanah to Hoshana Rabba, during which we atone and wipe our sins clean.
The numerical similarity between the periods requires that Three Weeks will also be holidays, how come they are days of sorrow? Except that these days are founded on days of rejoicing, the 17th of Tammuz should have been the day the Torah was received, on this date the tablets of the Ten Commandments were given, the 9th of Av should have been a day of rejoicing at the good news of entering the Land of Israel but because they slandered them they were prevented from entering Israel and it became a day of mourning for generations. This is why days of overwhelming pain and sorrow are attributed to days that were supposed to be joyous holidays for receiving the Torah and entering the Land of Israel.
They say "Serve the LORD with gladness." It is difficult to understand how the Sages command us to be sad and to be sorry during the Three Weeks since this apparently contradicts the commandment of perpetual joy.
To understand things, we will explain that the whole world consists of constant movement. Movement is the life force of the still and the entire universe. The direction of the movement is back and forth - right and left. Just as the world consists of movement between light and darkness, cold and heat - so the human body moves, as does the soul, quickly shifting moods: sadness and joy, hate and love, calmness and outburst, etc. These movements dangle from their higher source, in a movement between the Sefirot of Mercy (right) and Might (left).
Because of this, the Holy One, blessed be He, destroys His house and commands us to mourn for a while of our lives so that we will learn how to appreciate pleasure and joy, sacrifice and love in everyday life and in the future eternal life.
The Three Weeks are to be the interim days, the prophet Zechariah prophesies that the fasts instituted to commemorate the destruction of the first temple will become days of joy and happiness. "Thus saith the LORD of hosts: The fast of the fourth month, and the fast of the fifth, and the fast of the seventh, and the fast of the tenth, shall be to the house of Judah joy and gladness, and cheerful seasons; therefore love ye truth and peace." (Zechariah 8:19).
According to this source, it is understood that the fasts will be good days, and the Three Weeks will also become good days because originally these were holidays.
Rashi interpreted the verse "And you were only happy: it is not a command but a promise." Hinting that the Three Weeks from the 17th of Tammuz until Tisha Be'Av will become good days. That is why the Torah said and you were happy, those 21 days of the Three Weeks, which are lowly and limited in content, will become days of joy and good times.
(Torah excerpt from Rabbi Tsvi Schwartz and Rabbi Ytshak Gabay)
