When used as an adjective, it refers to peaceful days gone by, which sounds like a dream.
When used as a noun, it refers to a bird, both real (a kingfisher) and mythical. The mythical version of a halcyon once lived at sea and had the power to calm water and wind.
Ah, the Halcyon days of 1950s and 1960s science fiction, but the real world is harsh.
My influencers -
Most admired: Christ
Most admired pair: mom and dad
Most admired artists: composers Chopin, Tchaikovsky, Rachmaninoff, writers Chandler, Greene, Austen, Serling, directors Hitchcock, Zinnemann, Bresson, actors Bogarde, Bogart, Raft, Hayward, Stanwyck, Trevor, Windsor, tough types …
Most admired leaders: Julius Caesar, Churchill, Reagan, Pius XII
All but the first one were imperfect creatures.
Did any of them have Halcyon days?