There’s a difference between a prophecy that refers to “that day” and, for instance, the Apostle Peter’s use of “that great and terrible day of the LORD” (Acts 2:20 & 2 Peter 3:10). In the latter, Peter omits the adjectives “great” and “terrible” because he is addressing believers. The book of Isaiah uses the phrase “in that day” 18 times in reference to judgements which, after they had taken place, became history.
But whichever way we view such verses, we see the “day of the LORD” in a negative light. This is because we only associate the term with divine judgement.
But after a prophecy of judgement comes the promise of mercy, after the first in Isaiah 30:1-17 comes the second in Isaiah 30:18-26 (note the “in that day” of verse 23). Poetically, the light of the moon becomes as bright as that of the sun, and the light of the sun as that of seven days — “in the day when the LORD binds up the bruises of His people, and heals the stroke of their wound”.
So, yes, “in that day” will come darkness for those who have rejected the LORD, but that same day will see “sevenfold” daylight brighter than any in world history — the bright light of the day of Christ’s return.
So let’s look past the descending darkness and the increasing evil around us, to the soon coming “Light of the world”! Five times in the early chapters of Isaiah, the “hand of the LORD” is said to be “stretched out still” in judgements on the unrepentant. But the bondage that was on those who did repent was destroyed by God’s blessings (Isaiah 10:27).
We should likewise look beyond God’s “outstretched hand” of “that day” lesser judgements to The Great Day of Judgement, when every unsaved person will be called to answer for personal sins’ Our Lord Jesus Christ will preside as Judge. The redeemed will be present but only as onlookers.