According to the Wikipedia entry on halcyon:
A halcyon (pronounced /ˈhælsiən/) is a mythical bird—often identified as a kingfisher—said to breed in a floating nest at sea during the winter solstice, during which time it charms the wind and waves into calm. The term originates from the Greek myth of Alcyone. In popular use, can also mean to harken back to an earlier (usually fond) time.Two Greek myths feature a character named Halcyone, sometimes spelled Alcyone.
The first tells the story of seven nymphs--one of whom is called Halcyone--known collectively as the Pleiades. They are the daughters of the Titan Atlas and the sea-nymph Pleione. Following the defeat of the Titans in their war with the Olympians, Zeus condems Atlas to support the heavens on his shoulders. In the wake of Atlas's punishment, Orion, the giant huntsman, pursues the Pleiades. Zeus intervenes to give some comfort to Atlas, transforming the sisters first into doves and later into a constellation. Orion also becomes a constellation and continues to chase the sisters across the night sky. (Is it a good thing to be transformed into a constellation?)
Picture Credit: Sacred kingfisher; attributed to Fir0002/Flagstaffotos; used under the GFDL license which can be viewed at http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License,_version_1.2.