“in that day.” “That day” is the Day of Yahweh. See commentary on Zephaniah 3:11.
“Do not let your hands fall limp.” Letting the hand “fall limp” is idiomatic and based on the experience of being afraid and/or discouraged. Often when a person is physically scared, discouraged, confused, etc., they do not have the motivation and strength to get to work and deal with the situation they are in. Some English versions translate the idiom out of the text: “do not be discouraged” (NAB); “Don’t be afraid” (NLT); “Do not lose courage” (GWN). But being that specific in the translation leaves out all the other possibilities that a person’s hands might fall limp, so in this case it is best to leave the idiom in the text.