voice of ... day of ... Lord - that is, Jehovah ushering in that day with a roar of vengeance against the guilty (Jer 25:30; Amo 1:2). They who will not now heed (Zep 1:12) His voice by His prophets, must heed it when uttered by the avenging foe.
mighty ... shall cry ... bitterly - in hopeless despair; the might on which Jerusalem now prides itself, shall then fail utterly.