There are many idioms of improbability, or adynata, used to denote that a given event is impossible or extremely unlikely to occur.
In English
Events that can never happen
As a response to an unlikely proposition, "when pigs fly", "when pigs have wings", or simply "pigs might fly". "When Hell freezes over" and "on a cold day in Hell" are based on the understanding that Hell is eternally an extremely hot place. The "Twelfth of Never" will never come to pass. A song of the same name was written by Johnny Mathis. "On Tibb's Eve" refers to the saint's day of a saint who never existed. "If the sky falls, we shall catch larks" means that it is pointless to worry about things that will never happen.
Events that rarely or might never happen
"Once in a blue moon" refers to a rare event. In fact, a "blue moon" occurs every two to three years in a year that has 13 full moons instead of the more usual 12. The "blue moon" is the third full moon in a season having four full moons. "Don't hold your breath" implies that if you hold your breath while waiting for a particular thing to happen, you will die first.
Tasks that are difficult or impossible to perform
To have "a snowball's chance in Hell". "Like getting blood from a stone", and "like squeezing water from a stone". "Like finding a needle in a haystack" "Like herding cats"
Things that are impossible to find
"As rare as hen's teeth". "As rare as rocking-horse poo".
People or things that are of no use
"As much use as a one-legged man at an arse-kicking contest". "As much use as a chocolate teapot".