Charles Ellicott Commentary Lamentations 1:21

"They have heard that I sigh; there is none to comfort me; All mine enemies have heard of my trouble; they are glad that thou hast done it: Thou wilt bring the day that thou hast proclaimed, and they shall be like unto me." — Lamentations 1:21 (ASV)

They are glad that you have done it...—Historically, the words refer to the conduct of nations like the Edomites, as described in Psalms 137:7.

You will bring the day that you have called.—Better, proclaimed. By some commentators the first verb is taken as a perfect, You have brought, and the “day” is that of vengeance upon Judah. With the rendering of the Authorized Version, the clause coheres better with that which follows, and the “day” is that of the punishment of the exulting foes.