Where did the phrase hay day come from?

Webster's II New Riverside Dictionary defines Heyday -- A time of maximum power, prosperity, or popularity. "HEYDAY goes back to an Anglo-Saxon use of 'hey' as an expression of great enthusiasm and happiness. So a person's 'heyday' is the period of his greatest vigor and success." From "Morris Dictionary of Word and Phrase Origins" by William and Mary Morris (HarperCollins, New York, 1977, 1988).