# 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 day of his misfortune

"the time at which bad things happen to him"

# the day

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

# in the day of their destruction

"at the time when their enemies destroy them"