The only thing Chazal mandate for the 3 weeks is the 3 haftarot; the aveilut customs are later additions. The essence of the 3 weeks then is the messages we get from the Haftarot. The mourning customs are supposed to help us achieve that. What are these messages?
Rav Mayer Twersky – Tisha B’Av 5776: Haftarot for the 3 Weeks
August 3, 2016