only when; of all peopleOften emphasizes that something happens at an inconvenient or unexpected time, as in “only when...” or “of all times.” With a person, it can mean that person is especially unlikely to do something: 彼女に限って means “she, of all people.”
JA: 忙しい日に限って、急な仕事が入る。
EN: It’s always on busy days that unexpected work comes up.