Albert Barnes Commentary Zephaniah 3:18

"I will gather them that sorrow for the solemn assembly, who were of thee; [to whom] the burden upon her was a reproach." — Zephaniah 3:18 (ASV)

I will gather them who are sorrowful - for the solemn assembly, in which they were to “rejoice” (Leviticus 23:40; Deuteronomy 12:12, 18; Deuteronomy 16:11; Deuteronomy 27:7) before God and which in their captivity God made to cease.

They were of you (Lamentations 1:4; Lamentations 2:6), the true Israel who were grieved for the affliction of Joseph; to whom the reproach of it was a burden (Amos 6:6) (rather, ‘on whom reproach was laid’).

For this reproach of Christ is greater riches than the treasures of Egypt, and such shall inherit the blessing: Blessed are you, when men shall hate you, and when they shall separate you from their company, and shall reproach you and cast out your name as evil, for the Son of Man’s sake; rejoice you in that day, and leap for joy, for, behold your reward is great in heaven (Luke 6:22–23).