it'll be a cold day in hell
It is unlikely or impossible that it will ever occur. Used hyperbolically and often issued in defiance of some request or demand. It'll be a cold day in hell when I sign over the lease of my land to you. It will be a cold day in hell before the two parties agree to anything.
It'll be a cold day in hell (when something happens).
Rur. Something will never happen or is highly unlikely. It'll be a cold day in Hell when the city council agrees on where to build that bridge. It'll be a cold day in Hell when I forgive you.