The phrase "a cold day in hell" is an idiomatic expression used to describe a situation or event that is highly unlikely, implying that it would be extremely difficult for something to happen. Full definition
It will be a cold day in hell before sane intelligent Americans lets this sanctimonious loony become President! (religion.blogs.cnn.com)
just go ahead and make obama the dictator, all of you people are morons, afraid that you will be called racists, go ahead vote that Blk «A» hole back in so he can finish his destruction, but it will be a cold day in hell before I ever call obama my president or anything with respect, he can talk to the hand. (religion.blogs.cnn.com)
Soledad O'Brien should be ashamed of herself and, for me, it will be a cold day in hell before I watch CNN again. (religion.blogs.cnn.com)