Filmishmish

Filmishmish (في المشمش) is an Arabic term meaning "in the time of the apricots" or "when the apricots bloom", which is taken nonliterally to mean the equivalent of the English phrases "wishful thinking" or "when pigs fly."

It originates from the fact that the apricot picking season is just about two weeks, before it may start rotting on the trees. During those two weeks the schools used to close, and the entire village used to join hands in picking the apricots. So, if an Arab says that he will do something "during the apricot season", you then know that they will be too busy then, and it won't be done.