1961: heyday Apr 28, 2020

When one refers to something, or someone's heyday, this is used to denote the height of success in some way or another, but this only even became a noun recently. Indeed, this used to be an exclamation—not much unlike 'hooray'—as is still kept with the interjection 'hey', though the two are probably not related except for syllabic simplicity. It is generally held that 'heyday comes as a variation of 'high day', and other similar phrases from Dutch and German support this.

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