Hello , "When hell freezes over " " never in a million years" both are sarcastic responses to indicate that something is improbable or impossible to happen. Mentioning impossible things to happen ( like hell freezing or pigs flying ) as a condition for a specified event to occur ,means inclusively that it will never occur. Other similar expressions include " A snowball's chance in hell" , "A cold day in hell", "when pigs fly" .

when hell freezes over, never in a million years : means something never happen

But y'all should know that "Hell" freezes over all the time; it's the name of a town in Michigan. The History of Hell, Michigan (http://www.hell2u.com/hell_history.htm) Hell was first settled in 1838 by George Reeves and his family. George had a wife and 7 daughters. George built a mill and a general store on the banks of a river that is now known as Hell Creek. The mill would grind the local farmers grain into flour; George also ran a whiskey still, so a lot of times the first 7-10 bushels of grain became moonshine. In turn, horses would come home without riders, wagons without drivers. Someone would say to the wife, where is your husband? She’d shrug her shoulders, throw up her arms and exclaim, Ahh, he’s gone to Hell!” In 1841 when the State of Michigan came by, and asked George what he wanted to name his town, he replied, “Call it Hell for all I care, everyone else does.” So the official date of becoming Hell was October 13, 1841. Pretty funny, eh? :)