But I really have no idea where the word comes from or even if I do actually have the definition strictly correct. I'm a bit tired of talking turkey for now, so let's see where this one heads.
Well, this is the kind of thing that I find interesting. The original expression has nothing to do with days, apparently, but rather goes back to an expression from Middle English, heyda, which was an exclamation of playfulness or surprise, an extension of 'hey'. Heyda!--a bit like Hurrah! or Huzzah! This was back in the 1500s, and it wasn't until around the 1700s that it somehow got conflated with the word "day" and began to be used to describe the period of greatest vigor.
And here I was, thinking it had something to do with haymaking.
The Online Etymology dictionary notes that in Latin, hei was a cry of grief or fear, but heia was an "interjection denoting joy". Interesting what one little vowel sound will do.
Since this post is a bit shorter than my usual rambling, I'll put in a plug for a very nice little publisher out here on the West Coast, called Heyday Books , which does some good and sometimes very beautiful books on California and the West. I know of them partly because it's one of my jobs in the bookstore I work in to order backlist titles from them, but also because they have published a couple of really nicely made books by the late James D. Houston. Houston was probably best known for his novel Continental Drift, or perhaps the book he co-authored with his wife Jeanne Watasuki Houston, Farewell to Manzanar, which described her family's experiences in a Japanese interment camp.
Before he died, Heyday published a collection of essays he wrote on his sense of place, Where Light Takes Its Color From the Sea, which enjoyed quite a bit of success locally, and one that they've published posthumously, called A Queen's Journey, which is an unfinished novel about Liliuokalani, the last queen of Hawaii.
Heyda, Heyday! Heyda, James Houston!
(I just realized that I forgot to say why all the focus on James Houston in particular. It's because he was a Santa Cruz resident, a friend of the local bookstores, and really a pretty terrific person.)