The adjective halcyon is hard to use outside of the phrase halcyon days, and the reason has to do with its origin and development.
A halcyon is a sort of bird which we nowadays call a kingfisher.¹
The halcyon was supposed to have built her nest on or near the sea and to have incubated her eggs during the relatively peaceful weather surrounding the winter solstice.
The Greeks explained this period of calm by saying that the body of a young demigod, Ceyx, floated to shore after his ship was destroyed in a midwinter storm. He was found by his lover, Alcyone, daughter of the god of winds, who drowned herself in the sea. The gods took pity on the couple and reunited them, transforming them into kingfishers which make their nests in midwinter by the shore. Alcyone’s father quieted the winds every year to protect his daughter’s family.²
Since antiquity, halcyon days has meant this nesting time: a period of about two weeks of calm weather surrounding the winter solstice.³ The phrase has also come to be used as a metaphor meaning any calm, peaceful, or fruitful time. And out of it, in a sort of back formation, halcyon has transformed again, this time from a bird (a noun adjunct in the phrase halcyon days) into a separable adjective meaning “calm” or “peaceful”. But the transformation is not complete. Halcyon (adj.) still pairs mostly with words for time or space: for example, “days”, “years”, “air”, or “atmosphere”.⁴
Poetically speaking, the affinity is apt. Alcyone’s lover Ceyx was the son of the star Eosphorus, known today as Venus rising or the daystar.