On the first day of Menachem Av, the mourning for the destruction of the Holy Temple increases (until it culminates with the Fast Day of Tisha B’Av).
However, there is a mitzva (commandment) in the Torah to always be joyous. How does one integrate the ideas of mourning and rejoicing? The Lubavitcher Rebbe discussed this exact topic in a gathering and said:
“Simcha (joy) is a fundamental element necessary in the performance of all the Mitzvos. Each and every mitzva provides an opportunity for a Jew to connect and relate to G-d’s essence. The realisation that this opportunity exists generates true Simcha in the performance of all the Mitzvos. “In addition, however, since simcha must be internalised (to the point where the joy brings the individual to sing, dance, move his entire body, his hands and his feet), an attempt should be made to tie the observance of Torah and mitzvot to objects that naturally produce simcha. For that reason, though there were ample spiritual and philosophical reasons for simcha on the Jewish holidays, rather than rely on these influences, the Torah commanded that every Jew bring peace offerings (that were eaten later by him and his family), and drink wine on the festivals. The Talmud explains this principle, saying ‘Real simcha is enjoyed when feasting on sacrificial meat. Real simcha is enjoyed when drinking wine.’
“Similarly, though Torah study in general produces Simcha, it is necessary to find an object of study which itself naturally produces simcha. Hence, in these three weeks which are associated with the destruction of the Temple and also the Messianic redemption, it is appropriate to learn texts which concern themselves with the Redemption and likewise with the structure and measurements of the Temple. The study of these texts (and the realisation of the dearness of the Messianic Redemption which will result) will evoke genuine simcha. The fusion of the study of Torah and performance of the mitzvot with the quality of Simcha will hasten the coming of Moshiach and the re-entry of the Jewish people into the land of Israel.”