Plural of heyday; multiple periods of greatest success, popularity, or prosperity for a person or thing.
From heyday (probably from hey, an exclamation of joy, + day), originally meaning a period of youthful vigor, later generalized to any peak period of success.
The heyday of the Hollywood studio system, the heyday of radio, the heyday of vinyl records—every technology and culture seems to have a golden age where it absolutely dominated before something newer came along.
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