Plural of halcyon; kingfisher birds, or in poetic usage, symbols of peace and tranquility derived from Greek mythology.
From Greek mythology's Halcyone, later applied scientifically to the kingfisher genus Halcyon. The mythological meaning (peaceful days) comes from a legend about a goddess transformed into a kingfisher.
When ancient Greeks saw kingfishers nesting during winter storms, they thought the birds had magic powers that calmed the seas—hence 'halcyon days' became the English phrase for unexpectedly peaceful times.
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