do not gloat over

"do not be happy because of" or "do not take pleasure in"

your brother's day

Here "your brother" refers to the people of Israel because Jacob and Esau were brothers.

"the time at which bad things happen to him"

Each of the several occurrences of "day" refer to the time when Babylon attacked and destroyed Jerusalem.

"at the time when their enemies destroy them"

"at the time when they suffer"