What is rue the day?

"Rue the day" is an idiomatic expression used in English to signify regret or remorse for a particular event or decision. When someone says that they will "rue the day" they did something, it implies that they will come to deeply regret that action or decision in the future. The word "rue" itself means to bitterly regret or feel sorrow for one's actions or decisions. This expression is often used in literature, films, and everyday conversation to convey a sense of foreboding or inevitable regret.