This phrase is the same as the one found at the head of Zephaniah 3:11–13. Therefore, it marks off Zephaniah 3:16–20 as parallel to Zephaniah 3:11–13. That day is when the judgment against God’s people would be complete, and they will experience the fullness of God’s blessings.
16 On that day it shall be said to Jerusalem: “Fear not, O Zion; let not your hands grow weak.