(idiomatic) A period of calm, often nostalgic: “halcyon days of yore”, “halcyon days of youth”.
Origin of halcyon-days
From Latin Alcyone, daughter of Aeolus and wife of Ceyx. When her husband died in a shipwreck, Alcyone threw herself into the sea whereupon the gods transformed them both into halcyon birds (kingfishers). When Alcyone made her nest on the beach, waves threatened to destroy it. Aeolus restrained his winds and made the waves be calm during seven days in each year, so she could lay her eggs. These became known as the "halcyon days", when storms never occur.