heyday (n.) Look up heyday at Dictionary.com
late 16c., alteration of heyda (1520s), exclamation of playfulness or surprise, something like Modern English hurrah, apparently an extended form of Middle English interjection hey or hei (see hey). Modern sense of "stage of greatest vigor" first recorded 1751, which altered the spelling on model of day, with which this word apparently has no etymological connection.