"A party will be when hell freezes over."
Translation:Вечірка буде, коли рак на горі свисне.
The literal meaning in Ukrainian is: There will be a party when the lobster is whistling on the mountain.
The point of this saying is: when the thing that is impossible, or seems to be impossible , happens, then we will have a party.
In English, no one talks about lobsters on a mountain. Instead, people say, when hell freezes over.... , when pigs fly..., and other expressions.