The word "halcyon" ultimately derives from the Greek word alkyon, referring to the kingfisher.   In Greek mythology, the halcyon was a bird believed to have the power to calm the sea during its breeding season around the winter solstice. This period of calm seas became known as "halcyon days," which then expanded to mean a time of peace and prosperity in general.  This phrase originally referred to the period of calm weather around the winter solstice when the halcyon bird was believed to be nesting. Later, in the 16th century, "halcyon" began to be used as an adjective, particularly in the phrase "halcyon days," translating the Latin "alcyonei dies" and referring to the 14 days of calm weather around the winter solstice, and it was adopted from the Latin into Middle English as "alceon". 

According to the Oxford English Dictionary, the word "halcyon" was first used in English in the 14th century as a noun. It initially referred to the mythical kingfisher bird, and later evolved to describe a peaceful and calm period. The adjective form, signifying calm and peaceful, also appeared later in the 16th century. 

Examples of "halcyon" used in a sentence:  
"Flying around the Los Angeles area was still fun in those happy, halcyon days!" 
"It creates the foundation for a halcyon, innovation-driven investment expansion."
"During those early halcyon years the company's potentila for growth seemed unlimited." 
"Four decades later, PIR is taking a trip back to its halcyon days." 
""But war had somehow changed since the halcyon days of Tommy and Fritz." 
"This had been too halcyon a day to clutter up the memory of it with some backwoods bully's melodramatic bellowings." 
"A memento of youth, of halcyon days. But look deeper, beyond the frozen smiles, and there were stories behind the image."
"It seemed longer; or rather, it seemed as if a dark, deep abyss separated us from the halcyon days that had preceded the disaster."

Thank you @Bill W for this. I did not know and certainly would never have guessed a connection between halcyon and Kingfishers. I love learning new things about our words and the opportunity Word of the Day provides.

Just the facts:

HALCYON Definition & Meaning - Merriam-Webster

HALCYON Synonyms: 80 Similar and Opposite Words | Merriam-Webster Thesaurus

Websters 1828 - Webster's Dictionary 1828 - Halcyon