The phrase ‘Halcyon Days’ is usually used rather vaguely to refer — often regretfully and nostalgically — to a time when things were better than they are now, the sun always shone, etc. It is one of the countless words whose once highly specific meaning has been diluted by careless use; ‘Meld’ is another: it used to mean ‘declare’ or ‘announce’, especially in card games but now, probably because it sounds like a mixture of ‘Weld’ and ‘Melt’, most people use it to mean join, merge, or mix.
Anyway, ‘Halcyon Days’: there was an ancient belief that the kingfisher — Αλκυών in Greek — made a floating nest on the sea in the middle of winter, and for two weeks the benevolent gods kept the weather calm and bright while the eggs hatched. For some strange meteorological reason there is in fact in Greece a period of about two weeks of fine, warm, sunny weather in the middle of winter before the usual gloom, cold and high winds set in again. Here in the Northern Sporades we have lately been having Halcyon Days in that ‘proper’ older sense.