If you feel sometimes that your halcyon days are behind you, then don’t worry. If you’re in the northern hemisphere, you might be able to enjoy them soon. Well, in a classical sense, at least…
For you see, this expression has its origins in Ancient Greek mythology, and is, perhaps counter-intuitively, very much an expression of the winter. It comes from the story of Alcyone, daughter of Aeolus, god of the winds. When her husband Ceyx died in a shipwreck, the gods transformed the couple into halcyon birds, more commonly known in English as kingfishers.
When Alcyone was building her nest on a beach during the winter, waves threatened to destroy it, so Aeolus restrained his winds to protect it. He continued to do this each year, creating a period of calm during the winter in which no storms occurred, known as halcyon days.
The expression later came to refer to one’s glory days in the past, probably from the perspective of someone, during some particularly bad weather later in winter, looking fondly back at the earlier halcyon days.