The Halcyon Days are the 14 days around the winter solstice. According to Greek mythology, the halcyon, or European kingfisher, built its floating nest in mid-December. The ancients believed that the bird had the power to calm the waves while brooding her eggs, so 14 days of calm weather were to be expected around the winter solstice when the halcyon was nesting.
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. Grieving Alcyone was about to throw herself into the sea to join her beloved husband, but the gods took pity on the pair, transforming them into halcyons, with the power to still the stormy seas for 14 days near the time of the winter solstice while they hatched their young. For this reason, mariners credit the kingfisher, or “alcyon bird,” with the power to calm storms and raging seas.
Today, the phrase “Halcyon Days” has come to mean a sense of peace or tranquility. People often use the phrase to refer idyllically to a calmer, more peaceful time in their past.