Howl ye; for the day of the Lord is at hand (comp. Joel 1:15); literally, the expression used in both passages is a day of Jehovah. The idiom would not, however, allow the use of the article, so that the phrase is ambiguous.
"The day of Jehovah" is properly "that crisis in the history of the world when Jehovah will interpose to rectify the evils of the present, bringing joy and glory to the humble believer, and misery and shame to the proud and disobedient" (Cheyne).
But any great occasion when God passes judgment on a nation is called in Scripture "a day of the Lord." "a coming of Christ." And so here the day of the judgment upon Babylon seems to be intended. It shall come as a destruction from the Almighty.
Isaiah is thought to quote from Joel (Joel 1:15) here; but perhaps both prophets quoted from an earlier author. Shaddai (equivalent to "Almighty') is an ancient name of God, most rarely used by the prophetical writers (only here, and in Ezekiel 1:24; Ezekiel 10:5; Joel 1:15), and never elsewhere by either Isaiah or Joel.
It has generally been said to mean "the Strong One;" but recently the theory has found favor that it meant originally "the Sender of storms," from the Arabic sh'da—jecit, effudit. However this may be, the word is certainly used in the later times mainly to express God's power to visit and punish, and the present passage might perhaps be best translated, "It shall come as a destruction from the Destroyer (k'shod mish-Shaddai yabo')."