Having dealt with the Jews" oppressors (cf. Zephaniah 3:8-15; Zephaniah 2:4-15; Genesis 12:3), the Lord will deliver even the weak and dispersed of His people and give them a worldwide reputation for goodness (cf. Deuteronomy 26:19). He will regather them in their land and give them a good reputation when He restores their fortunes (cf. Zephaniah 3:15; Genesis 12:1-7; Genesis 13:14-17; Genesis 15:7-21; Genesis 17:7-8; 2 Samuel 7:16; Psalm 89:3-4; Isaiah 9:6-7; Daniel 7:27). [source][source][source]
Zephaniah concluded his book by affirming that such was Yahweh"s declaration. He would indeed restore His people. [source][source][source]
"The whole message of Zephaniah is finally united in one grand inclusio, in that it begins and ends with Yahweh, Israel"s just but caring covenant God, whose word ( Zephaniah 1:1) is spoken ( Zephaniah 3:20)." [1][source]
An inclusio is a repetition of key elements, either words or motifs, at the beginning and end of a literary unit. [source]
Eight times in Zephaniah 3:18-20, in the NASB, the Lord said, "I will," "I am going to," or "When I." The future restoration and blessing of Israel in the world will be something that Yahweh Himself will accomplish "in that day" (i.e, the day of the LORD). No one but He could ever accomplish it, and no one but He would. [source][source][source]