Heyday refers to the period of one's greatest popularity, vigor, or prosperity. It is usually used in the singular.

// In its heyday, the circus was a major form of entertainment for the small town.

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"In its heyday, there were more than 200 Chi-Chi's nationwide; the last restaurant closed in 2004." — Nicole Hvidsten, The Minnesota Star Tribune, 1 Oct. 2025

The day in heyday originally had nothing to do with the kind of day that's made up of 24 hours. Heyday was first used in the first half of the 16th century as an extended form of the interjection hey, used since the 13th century to express elation or wonder, as it still often is in phrases like "hey, look at that!" The day part was most likely just an extra syllable tagged on for effect. By the end of the 16th century heyday had developed noun use with the meaning "high spirits," as when Shakespeare's Hamlet tells his mother, "You cannot call it love; for at your age / The heyday in the blood is tame …” It wasn't until the 18th century that the day syllable's resemblance to the word day likely influenced the development of the now-familiar use referring to the period when one's achievement or popularity has reached its zenith.