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 (plural heydays)
- A period of success, popularity or power; prime.
- The early twentieth century was the heyday of the steam locomotive.
Related terms 
- A lively greeting.
- 1798:"Heyday, Miss Morland!" said he. "What is the meaning of this? I thought you and I were to dance together." Jane Austen - Northanger Abbey
- (obsolete) An expression of frolic and exultation, and sometimes of wonder.
- (Can we find and add a quotation of Ben Jonson to this entry?)