Perhaps alteration ofheydaexclamation of pleasureprobably alteration of Middle English heyhey
American Heritage Dictionary of the English Language, 5th Edition
Late 16th century, from earlier heyda (1520s), as exclamation – compare hey, hei. Sense “period of success, vigor” from 1751, which respelt as heyday based on unrelated day (as “period of time”) – compare day in the sun.
Heyday Sentence Examples
In the heyday of his youth his high spirits and passion for adventure enabled him to surmount every obstacle with elan.
In the heyday of the Athenian democracy, citizens both conservative and progressive, politicians, philosophers and historians were unanimous in their denunciation of "tyranny."
Douglas, then still in the heyday of his popularity.
The face on important occasions is usually much painted, save by young ladies in the heyday of beauty.
In the heyday of his passion for Fraulein von DOnniges, his dream was to be enthroned as the president of the German republic with her seated at his side.