Peaceful or Happy…
An old man watching his grandchildren play might look back fondly on his halcyon days, remembering the peaceful, happy time of his youth.
The word halcyon comes from a story in Greek mythology about the halcyon bird, which had the power to calm the rough ocean waves every December so she could nest. Like those calm waters, halcyon has come to mean a sense of peace or tranquility. People often use the phrase halcyon days to refer idyllically to a calmer, more peaceful time in their past.
Thanks to Vocabulary.com