The phrase “halcyon days” refers to the seven days preceding and the seven days following the shortest day of the year, believed by the ancients always to be windless and calm; hence, any protracted time of peace and serenity, rest and rejoicing.
According to Greek legend, Haleyone and her husband, whom she had found drowned upon the shore, were turned into birds by the gods and were thereafter known as halcyons, or, as we call them, kingfishers.
Their nests were believed to be built upon the sea, and the gods decreed that whenever these birds wished to build their nests, the sea should remain perfectly calm and unruffled. They made their nests, according to Pliny, during the seven days preceding the winter solstice and brooded upon their eggs during the next seven days.
The same writer says that the nests, as they floated upon the water, resembled a kind of ball, not unlike a large sponge. The legend, of course, and Pliny’s description are equally fanciful, but they were believed as late as 1398 when.
John de Trevisa, in translating De Proprietatibus Rerurn by Bartholomaeus, wrote, “In the cliffe of a ponde of Occean, Alicion, a see foul; in wynter maketh her neste and layeth egges in vii days and sittyth on brood seuen dayes.”