Adam Clarke on Zephaniah 3:18

I will gather - sorrowful - This may refer to those who, during the captivity, mourned for their former religious assemblies; and who were reproached by their enemies, because they could not enjoy their religious solemnities. See Psa 137:1-9 : "By the rivers of Babylon, there we sat down, yea, we wept, when we remembered Zion. For there they that carried us away captive required of us a song," etc. This very circumstance may be the reference here.