Today (or possibly yesterday, but I forgot) is the first of the Halcyon Days which will last until just before the end of December. The Halcyon Days are the fortnight of supposedly calm weather in midwinter during which the halcyon, or kingfisher, lays her eggs.
The eminent biologist/meteorologist Ovid explains that Ceyx and Alcyone were lovers. Ceyx had to go to sea for some reason and every day Alcyone (the girl) would go down to the shore to look for his returning vessel. She continued this vigil until she was informed by the utterly reliable medium of a dream that the ship had sunk and Ceyx had drowned. At this news, as Chaucer so movingly put it,
"Allas!" quod she for sorwe
And deyede within the thridde morwe.
[quod=said sorwe=sorrow deyede=died thridde=third morwe=day]
Anyway, everybody was terribly upset including the gods who decided on their tried and tested fall-back plan of turning both the lovers into birds (avification was only just behind stellafication in the Attic list of divine mercies).
The gods then went further and decided to make the sea calm and the weather bright for one fortnight a year starting on the 14th/15th of December so that the Alcyon could explore and develop her newfound egg-laying capabilities, for which I'm sure the young lady was profoundly grateful.
Anyway, the usual modern sense of the halcyon days as being the happy days before all the trouble started is a figurative extension of this precise ornithometeorlogical usage.
Note the wings forming