Chapter 3 ends with a rebuke a rebuke to the women of Jerusalem (verses 16 & 17), and verse 18 begins the judgment that will be poured out, beginning with the phrase “On that day.” Through the end of chapter 3, all the wealth and signs of wealth will be stripped away from them with this result:
Isaiah 3:25-26 (HCSB) “Your men will fall by the sword, your warriors in battle. Then her gates will lament and mourn; deserted, she will sit on the ground.”
This judgment on the women continues in what we call chapter 4, beginning with the phrase “On that day;” as seven (symbolizing the complete number of Hebrew women) seek one man to provide for them and take away their disgrace.
Then we read, Isaiah 4:2-6 (HCSB) “On that day the Branch of the LORD will be beautiful and glorious, and the fruit of the land will be the pride and glory of Israel’s survivors. Whoever remains in Zion and whoever is left in Jerusalem will be called holy—all in Jerusalem who are destined to live— when the Lord has washed away the filth of the daughters of Zion and cleansed the bloodguilt from the heart of Jerusalem by a spirit of judgment and a spirit of burning. Then the LORD will create a cloud of smoke by day and a glowing flame of fire by night over the entire site of Mount Zion and over its assemblies. For there will be a canopy over all the glory, and there will be a booth for shade from heat by day, and a refuge and shelter from storm and rain.”
“On that day” promises are fulfilled and Isaiah uses familiar language to communicate to the Hebrew people how glorious it will be when the Lord washes away their filth and cleanses them of the bloodguilt of their wretched hearts. The familiar signs of a cloud of smoke and flame by night hearken back to the wilderness; the booth for shade reminds me of the end of Jonah’s account. Isaiah is pointing the Hebrews forward to the age-to-come, where the survivors of Israel (a remnant, Paul would say) are forgiven and live with YHWH in fellowship.