In English, we often use the term "when pigs fly" to indicate something that will never happen. It's become so common that we've managed to make flying pigs a pop culture meme. It's not the only way to say it in English, and other languages have their own colorful phrases for "not gonna happen." For example:
Israel "When hair grows on the palm of my hand"
Hungary “When it’s snowing red snowflakes”
Portugal “In the afternoon of Saint Never’s day”