"Hell Hath Frozen Over": A Figure of Speech, Not an Event

The phrase "Hell hath frozen over" (or "Hell freezes over") is an idiom meaning "something extremely unlikely has happened." It is a hyperbolic expression of surprise, not a reference to a real event. There is no year, date, or historical occurrence associated with it.

Examples of use:

• "He finally apologized—hell hath frozen over!"
• "When the Cubs win the World Series, hell will freeze over." (They did in 2016; hell did not freeze.)

It likely originates from the traditional Christian concept of Hell as a place of eternal fire; its freezing would be a miracle/impossibility. Similar: "pigs fly," "when the moon turns blue."