When someone says "It will happen when hell freezes over", it means "It will never happen..."
Unless it does happen.
When people asked Don Henley whether the Eagles would get back together, Henley would always say "We'll get back together when hell freezes over." Little did they know 14 years later, they would get back together, and they immediately knew what to call their new live album.
Idiomaticexpression used to describe an event or scenario that is so unlikely that it will likely never occur.
Q: Dude, I'm tired of the beach. Wanna go and ride that fake, plastic wave that they setup at Wal-Mart?
A: Yeah!
Q: Right on! Let's roll!
A: Hold on, I'd rather do that when hell freezes over. You stupid nincompoop.
"Rita, when are you going to give me a kiss?" asked Beta. Rita responds with "to the day hell freezes overwhile walking out the door of the room" leaving Beta depressed.