a cold day in hell

"A cold day in hell" is a figurative phrase used to describe something that is highly unlikely or improbable to happen. It refers to the idea that hell, being traditionally associated with intense heat and fire, would never experience a day cold enough for snow or extreme cold temperatures. Thus, the phrase implies that the event being referred to is so unlikely that it would only occur in a scenario that is considered impossible or highly unlikely, similar to a cold day in hell.