Inspired by a similar thread in the German forum I would like to ask what expressions your language has to express the idea of the day that will never come. -Will you go out with me? -Yeah, wenn hell freezes over. The Russians have "после дождика в четверг" (when it rains on Thursday). I've read it comes from the fact that people would pray to Perun, the god of thunder, asking to send them rain. It was mostly done on Thursday, the day of the week dedicated to Perun. Naturally, the prayers were all too often unsuccessful, which gave birth to the abovementioned expression.