In Greek mythology Ceyx, son of Eosphorus and the king of Thessaly was married to Alcyone. They were very happy together, but after Ceyx perished in a shipwreck, Alcyone threw herself into the sea. Out of compassion, the gods changed them into the halcyon birds. 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. The halcyon has become a symbol of tranquillity. (Ovid MetamorphosesXI, 410).

The Departure of Ceyx, Vittore Carpaccio

Bulfinch : Ceyx and Halcyone: or, the Halcyon Birds

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Ceyx, lord of Trachis, was connected by friendship with Heracles. He was the father of Hippasus, who fell in battle fighting as the ally of Heracles. (Apollod. ii. 7. § 6, &c.) According to others, Ceyx was a nephew of Heracles, who built for him the town of Trachis. Müller (Dor. ii. 11. § 3, comp. i. 3. § 5) supposes that the marriage of Ceyx and his connexion with Heracles were subjects of ancient poems..