Charles Ellicott 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)

The festival of the accomplishment of salvations is represented by the figure of the joyous Feast of Tabernacles, as in Zechariah 14:16. No one will be prevented from attending this joyous occasion, for the oppressors will be overthrown (Zephaniah 3:19–20).

To whom the reproach of it was a burden. —Or, on whom reproach was a burden — that is, those on whom their exile, and resulting inability to attend at Jerusalem, had brought derision. Regarding the grammatical construction, the Hebrew student can consult Hitzig or Kleinert.