When commemorating loss, we use the halachic system of aveilut, mourning. Fasting is not part of that system, yet when it comes to the four days associated with the loss of the Beit HaMikdash, we don’t only mourn, we fast! Why? We also see, from Zechariah that in the future, when the Beit HaMikdash is rebuilt, the days won’t lose their significance, become ordinary days; they will turn into days of joy. Why?
Recorded at OU Israel’s L’Ayla Rosh Chodesh Women’s Event