For a selection of uses of the phrase "halcyon days" see web address 1 below. Note that the initial h-, used in English from Shakespeare's day to our own but not in other languages such as French, is not found here or in most Greek sources (but is found in our texts of some ancient grammarians). It comes from an apparently false etymology of these words from the root for salt, hal- (see alpha 1300). The kingfisher was well known for its mournful, lamenting cry, usually derived from the death and metamorphosis of Alkyone (see alpha 1299), but here associated with the leader of the Giants in the Gigantomachy. Its cry was taken as one of ill-omen (e.g. Apollonius Rhodius, Argonautica 1.1085).
[1] cf. the scholia to Aristophanes, Birds 250.
[2] The halcyon days were associated with a habit of nesting and incubating for 7 days on either side of the winter solstice, a total of 14 days, described first by the poet Simonides in a fragment (PMG 508 Page) from his Victory Odes for the Pentathlon (this poet was probably the first to try to immortalize in grand choral odes the transitory glory of athletic victory), then by Aristotle -- quoting Simonides -- in History of Animals 542b4-24. During those days, especially around Sicily, the seas were calm and the weather clear; hence the phrase. Aristotle's description is the basis of the present entry and those in lexica by Pausanias the Atticist, Photius, and others.
[3] Demagoras fr. 3 FHG (4.378); Philochorus FGrH 328 F186. (For Philochorus see phi 441 and generally OCD(4) s.v.) The numbers in the text of the Suda are corrupt and must be emended: the figure in Simonides and Aristotle is unambiguously 14.
[4] Hegesander (or Hegesandros) of Delphi compiled, in the C2 BC, at least 6 books of unreliable Memoirs, often cited by Athenaeus; see OCD(4) s.v. (This is his fr. 46 FHG = 4.422.) Some authorities believe that the nest floated on the calm sea for those days, but others were aware that the kingfisher nests in the sand. Lucian in his True Histories tells of an imaginary giant kingfisher, but the ancients seem to have thought of the real kingfisher, beloved by Zeus, who calms the weather for its nesting (Simonides). The Alcyon Sea was so named, according to Strabo, because this corner of the Gulf of Corinth is both a nesting place for the kingfisher and a stretch sheltered from the wind.
[5] cf. phi 672.
[6] cf. alpha 2505.
[7] cf. mu 434.
[8] cf. already alpha 1287.
[9] cf. pi 79, where the name is (correctly) "Pallene".
[10] cf. delta 1529.
[11] cf. alpha 4231.
[12] In the Greek, *)alkuo/neios (almost unattested outside lexicography) rather than *)alkuoni/s.