Chassidus explains that the greatest joy is from light that comes after darkness.
This idea can be found in connection with all 5 things.
Generally, 15 Menachem-Av symbolizes the inner light and hope that is concealed in golus.
This is especially symbolized by the full moon of the month of Av. Of course, the moon is always full, but on 15th, the light shines brightly. That is why ישראל דומין ללבנה – sometimes our light is concealed. The greatest concealement is on Tisha B’av, and therefore, the full moon of the month of Av is the greatest joy and nechama.
It reminds us that despite being in the darkest golus, we will soon shine again with the coming of Moshiach.
This is also stressed by the idea of כלו מתי המדבר: in truth, the Yidden stopped dying on Tisha B’av, but they only knew for certain on 15 Av. So too, Moshiach was already born on Tisha B’av, but the revelation and light of geula is specifically connected to 15 Av.
Gut Yom Tov!
May we be zoche to celebrate this 15 Av with the light of Moshiach in the most revealed way!