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1540s, in halcyon dayes (Latin alcyonei dies, Greek alkyonides hemerai), 14 days of calm weather at the winter solstice, when a mythical bird (identified with the kingfisher) was said to breed in a nest floating on calm seas. From halcyon (n.), late 14c., from Latin halcyon, from Greek halkyon, variant (perhaps a misspelling) of alkyon "kingfisher," from hals "sea, salt" (see halo-) + kyon "conceiving," present participle of kyein "to conceive," literally "to swell," from PIE root *keue- "to swell." Identified in mythology with Halcyone, daughter of Aeolus, who when widowed threw herself into the sea and became a kingfisher.