When something’s in its heyday, its in its prime. What does that have to do with hay? Nothing, actually. It goes back to the 1500s, when heyday and similar-sounding words were simply expressions of celebration or joy. Grant is especially fond of the Oxford English Dictionary‘s first citation for this term, from the John Skelton’s Magnyfycence, published around 1529: “Rutty bully Ioly rutterkin heyda.” This is part of a complete episode.