People often say “on a cold day in Hell” or “when Hell freezes over” as if that would be literally the end of the world. Contrary to popular belief, other planets and even other dimensions have wacky weather too – not just this version of Earth! However, Hell is notorious for its severe heat and its any-color-but-white Christmases (if you can even call them that), so the expression does correctly indicate a high degree of rarity or unlikelihood.
