it will be a cold day in hell

It will be a cold day in hell is another way of saying something is never going to happen or that it is very unlikely. As hell is supposed to be a very hot place, literally, with fire burning constantly; it would certainly not be cold. If it was a cold day in hell, then it is under the most unforeseen circumstances. When someone says it will be a cold day in hell, they mean it is highly unlikely and would be shocking if it happened.