The time of triumph and exultation; hence, joy, high spirits, frolicsomeness; wildness
Someone's heyday is the time when they are most powerful, successful, or popular. In its heyday, the studio's boast was that it had more stars than there are in heaven. the time when someone or something was most popular, successful, or powerful in sb's heyday (heyda a shout of happiness (16-17 centuries); influenced by day)