There are some other fun English phrases with the same meaning, i.e., snarky-toned "that's never gonna happen." For example, "Yeah we can grab dinner...when pigs fly."
well, as we say sometimes "коли свині літатимуть")))
Duolingo should accept "A party will happen when hell freezes over," right? "A party will be when hell freezes over" sounds very foreign.
Yeah, I agree that this sounds funky. I would translate it as "There will be a party when hell freezes over."