/ ˈheɪdeɪ; NAmE / noun
[ usually sing. ] the time when sb/sth had most power or success, or was most popular
SYN prime :
In its heyday , the company ran trains every fifteen minutes.
a fine example from the heyday of Italian cinema
a picture of Brigitte Bardot in her heyday
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WORD ORIGIN
late 16th cent. (denoting good spirits or passion): from archaic heyday! , an exclamation of joy, surprise, etc.