Halcyon is a name for a bird of Greek legend which is commonly associated with the kingfisher. The phrase comes from the ancient belief that fourteen days of calm weather were to be expected around the winter solstice - usually 21st or 22nd of December in the Northern Hemisphere. as that was when the halcyon calmed the surface of the sea in order to brood her eggs on a floating nest. The Halcyon days are generally regarded as beginning on the 14th or 15th of December.
Halcyon means calm and tranquil, or 'happy or carefree'. It is rarely used now apart from in the expression halcyon days. The name of the legendary bird was actually alcyon, the 'h' was added in regard to the supposed association with the sea ('hals' in Greek).
The source of the belief in the bird's power to calm the sea originated in a myth recorded by Ovid. The story goes that Aeolus, the ruler of the winds, had a daughter named Alcyone, who was married to Ceyx, the king of Thessaly. Ceyx was drowned at sea and Alcyone threw herself into the sea in grief. Instead of drowning, she was carried to her husband by the wind. The rest of the story is, in a translation of Ovid: