Refers to the period when someone or something is at its peak of power, popularity, or prosperity.
His thirties were his heyday as an athlete.
この町は江戸時代に全盛期を迎えた。
This town reached its golden age in the Edo period.
He reached his prime in his early thirties.
That band's heyday has already passed.
At his peak, he never lost to any opponent.
That company once boasted the industry's greatest heyday.