heyday (n.)

also hey-day, late 16c. as an exclamation, an alteration of heyda (1520s), an exclamation of playfulness, cheerfulness, or surprise something like Modern English hurrah; apparently it is an extended form of the Middle English interjection hey or hei (see hey). Compare Dutch heidaar, German heida, Danish heida. Modern sense of "stage of greatest vigor" first recorded 1751 (perhaps from a notion that the word was high-day), and it altered the spelling.