heyday

UK: /ˈheɪdeɪ/ US: /ˈheɪˌdeɪ/ ,(hā′dā′)

WordReference Random House Learner's Dictionary of American English © 2026

hey•day¹ /ˈheɪˌdeɪ/   n. [countable* usually singular]

1. the stage or period of greatest strength, life, success, etc.;
   prime:
   the heyday of the silent movies.

WordReference Random House Unabridged Dictionary of American English © 2026

hey•day¹  (hā′dā′), n. 

1. the stage or period of greatest vigor, strength, success, etc.;
   prime:
   the heyday of the vaudeville stars.
2. [Archaic.] high spirits.

Also, hey′dey′. 

• variant of high day, apparently by confusion with heyday² 1580–90

hey•day²  (hā′dā), interj. [Archaic.]

1. (used as an exclamation of cheerfulness, surprise, wonder, etc.)

• German hei da hey there
• 1520–30; rhyming compound based on hey; replacing heyda

Collins Concise English Dictionary © HarperCollins Publishers::

heyday / ˈheɪˌdeɪ/ noun

1. the time of most power, popularity, vigour, etc; prime

Etymology: 16th Century: probably based on hey