‘And in that day the Lord, Yahweh of hosts, called to weeping and to mourning, and to baldness and to girding with sackcloth.'
For there was One Who had offered another way. That One was the sovereign Lord, Yahweh the great ‘He is', the Deliverer from Egypt, the One Who was over all the hosts of heaven and earth. And this way was Yahweh's way of victory. Let them but come to Him in repentance, in mourning over sin, in weeping over their idolatry and the way that they had neglected Him and His Law (compare here Joel 2:12-17). Let them genuinely repent and demonstrate it by the outward signs of weeping and mourning by shaving their hair (Jeremiah 47:5; Amos 8:10) and wearing sackcloth to prove their genuineness, and then God would hear them and they would be delivered. But it had to be genuine. (Contrast here Isaiah 58:2-14 which describes the kind of fasting that would not have been pleasing to Him).