When Messiah establishes His Messianic Kingdom, God’s promises to Israel will be accomplished and the mourning associated with Tisha B’Av will no longer be relevant.
“This I recall to my mind, therefore I have hope. Through the Lord’s mercies we are not consumed, because His compassions fail not. They are new every morning; great is Your faithfulness. ‘The Lord is my portion,’ says my soul, ‘Therefore I hope in Him!’” Lamentations 3:21-24