Your heyday is the stretch of years when you — or a thing — were at your strongest: most loved, most successful, or most powerful. You nearly always meet it in a little frame: in her heyday, in the heyday of radio. It often looks back, with a fond, slightly sad feel, as if the best time is already behind. People do also use it for a peak that is happening now, but the looking-back use is the one you will hear most. Though actually, it did not start as a time-word at all. It began as a happy shout, like hey!, written heyda in the 1500s. Later writers respelt it heyday because day made it sound like a period of time — the same idea as a day in the sun. The day in the spelling is not really the word day; it only looks that way. You may still see the old shout in very old books; in living English the word is almost only this “best years” sense.